Orhan in early stages of his reign was occupied by the conquest of most of northwestern Anatolia. Most of these areas were under Byzantine rule and he won the first set battle, Battle of Pelekanon against the Byzantine Emperor Andronikos III Palaiologos. He also occupied the lands of Karesi Principality ruled by a Turkish Emir and of Ankara which was under the religious fraternity-guilds called Ahis.
During the last years of the civil war in the Byzantine Empire during 1341-1347 between the guardians of the young Emperor John V Palaeologus and the usurper John VI Cantacuzene, Orhan was induced to marry Theodora, daughter of Cantacuzene, in order to support him to become the ruling Emperor.
In 1354 Orhan's son, Suleyman Pasha (Süleyman Paşa), occupied Gallipoli (evacuated by its Byzantine population in the wake of an earthquake) and gave the Ottoman state a bridgehead into mainland Europe.
When Orhan succeeded his father, he proposed to his brother, Alaeddin, that they should share the emerging empire. The latter refused on the grounds that their father had designated Orhan as sole successor, and that the empire should not be divided. He only accepted as his share the revenues of a single village near Bursa.
Orhan then told him, "Since, my brother, thou will not take the flocks and the herds that I offer thee, be thou the shepherd of my people; be my Vizier." The word vizier, vezir in the Ottoman language, from Persian wazīr, meant the bearer of a burden. Alaeddin, in accepting the office, accepted his brother's burden of power, according to oriental historians. Alaeddin, like many of his successors in that office, did not often command the armies in person, but he occupied himself with the foundation and management of the civil and military institutions of the state.
According to some authorities, it was in his time, and by his advice, that the practices of assemblance of vassalage to the ruler of Konya, stamping money with his effigy, and using his name in public prayers, was discontinued by the Ottomans.
These changes are more correctly referred by others to Osman himself, but the vast majority of the oriental writers concur in attributing to Alaeddin the introduction of laws, which endured for centuries, respecting the costume of the various subjects of the empire, and of laws which created a standing army of regular troops, and provided funds for its support. It was by his advice and that of a contemporary Turkish statesman that the celebrated corps of Janissaries was formed, an institution which European writers erroneously fix at a later date, and ascribe to Murad I.
Orhan's predecessors, Ertughrul and Osman, had made war at the head of the armed vassals and volunteers. This army rode on horseback to their prince's banner when summoned for each expedition, and were disbanded as soon as the campaign was over. Alaeddin determined to ensure and improve future success, by forming a corps of paid infantry, which was to be kept in constant readiness for service. These troops were called Yaya, or piyade. They were divided into tens, hundreds, and thousands with their commanders Their pay was high, and their pride soon caused their sovereign some anxiety. Orhan wished to provide a check to them, and he took counsel for this purpose with his brother Alaeddin and Kara Khalil Çandarlı (of House of Candar), who was connected with the royal house by marriage. Çandarlı laid before his master and the vizier a project. Out of this arose the renowned corps in the Janissaries, which was considered the scourge of Balkans and Central Europe for a long time, as well as the terror of their own sovereigns, until it was abolished by Sultan Mahmud II in 1826.
Çandarlı proposed to Orhan to create an army entirely composed of the children of conquered places. Candarli argued that:
He also claimed that the formation of Janissary out of conquered children would induce other people to adopt, not only out of the children of the conquered nations, but out of a crowd of their friends and relations, who would come as volunteers to join the Ottoman ranks. Acting on this advice, Orhan selected out of the families of the Christians whom he had conquered, a thousand of the finest boys. This enrollment, training and employment the recruits as to their individual abilities to posts ranging from ranks of a professional soldier to the highest office in the Ottoman Empire as the Grand Vizier, was continued for centuries, until the reign of Sultan Mehmet IV., in 1648.
Orhan, with the help of Ghazi commanders at the head of his forces of light cavalry, started a series of conquests at the Northwest Anatolia of places that were part of Byzantine Empire. First in 1321 Mudanya was captured on the Marmara Sea, which was the port of Bursa. He then sent a column under Konur Alp towards West Black Sea coast; another column under Akcakoca to capture the peninsular land west of Izmit (Nicomedia) which the Turks call Kocaeli and finally a column to capture southeast coast of Marmara Sea. Then, he started captured the city of Bursa with just diplomatic negotiatioms. The Byzantine commander of the Bursa fort, called Evronos Bey, became a commander of a light cavalry force and even his sons and grandsons sereved Ottoman Empire in this capacity to conquer and hold many areas in Balkans. Once the city of Bursa was captured, then Orhan Bey sent cavalry troops towards Bosphorus capturing Byzantine coastal towns of Marmara. There were even sightings of Ottoman light cavalry alaong the Bosphoros coast.
The Byzantine Emperor Andronicus III gathered together a mercenary army and set of towards Anatolia on the peninsular lands of Kocaeli. But at the present towns of Darica, at a site then called Pelekanon, not too far from Uskudar, he met with Orhan's troops. There was a battle between the two forces and Byzantine forces were routed by Orhan's disciplined troops. Thus, in 1329 after this Battle of Pelekanon the Byzantines gave up the idea of getting the Kocaeli lands back and never tried conducting a field battle against the Ottoman forces.
The city of Nicaea (second only to Constantinople in the Byzantine Empire) surrendered to him after a three-year siege in 1331. The city of İzmit or Nicomedia was also captured in 1337. Orhan gave the command of it to his eldest son, Suleyman Pasha, who had directed the operations of the siege. In 1338 by capturing Uskudar most of Northwest Anatolia was in Ottoman hands. The Byzantines still held direct control over the coastal strip from Sile on Black sea to Uskudar and the city of Amasra (Amastris) on Paphlagonia but these were scattered and isolated to cause no threat to Ottomans.
Then, there was change of strategy in 1345 and instead of aiming to gain land from non-Muslims, a Turkish principality, Karesi (present Balikesir and surrounds) was taken over. According to Islamic philosophy of war, the areas under Islamic rule were to be abodes of peace and the other areas abodes of war. In abodes of war conducting a war was considered a good-deed. Karesi principality was a state governed by a Turkish Emir and its main inhabitants were Turkish; so it was a part of abode of peace. Ottomans had to have special reasons why they would like to conquer fellow Muslim Turkish principalities. In the case of Karesi principality, the ruler had died and had left two sons whose claims to the post of Emir was just as valid. So there was a fight between the armed supporters of the two claimant princes. Orhan's pretext for invasion was that he was acting as a bringer of peace. In the end of the invasion by Ottoman troops the two brothers were pushed to the castle of their capital city of Bergama (Pergamum). One was killed and the other was captured. The territories around Bergama and Balikesir (Palaeocastro) were annexed to Orhan's domains. This conquest was particularly important since it brought Orhan's territories to Canakkale, the Anatolian side of the Dardanelles Straits.
On the conquest of Karasi, in the year 1336, nearly the whole of the North-West of Anatolia was included in the Ottoman Empire, and the four cities of Bursa, Izmit (Nicomedia), Iznik (Nicea), and Bergama (Pergamum) had become strongholds of its power. At this stage of his conquests Orhan's Ottoman Principality had four provinces:
A period of twenty peaceful years followed the acquisition of Karasi. During this time, the Ottoman sovereign was actively occupied in perfecting the civil and military institutions which his brother had introduced, in securing internal order, in founding and endowing mosques and schools, and in the construction of vast public edifices, which still stand. Orhan did not continue with any other conquests in Anatolia except taking over Ankara from the commercial-religious fraternity guild of Ahis.
The general diffusion of Turkish populations over Anatolia, before Osman's time, was in main part a push from the Mongol conquest of Central Asia, Iran and then East Anatolia. They had founded a number of Turkish principalities after the demise of the Anatolian Sultanate of Rum, after its defeat by the Ilkhanate Mongols at the Battle of Kose Dag in 1243. These principalities, including the Ottomans under Orhan until reign of his son Murat I, still were tributary states paying yearly tribute to the Ilkhanids in Persia. Although they were all of Turkish stock, they were considered to be rivals in attempts to obtain dominant status in Anatolia.
The friendly relations which Orhan formed with the Andronicus III Palaeologus after the Byzantine defeat of Battle of Pelekanon and maintained (though not without interruption) with that emperor and some of his successors, caused a long twenty year period of general repose to the Ottoman power.
However, as the civil war of 1341-1347 dissipated the last resources of the Byzantine Empire, the auxiliary armies of the Emirs of Turkish principalties were frequently called over and employed in Europe. In 1346, The Emperor John VI Cantacuzene recognised Orhan as the most powerful sovereign of the Turks. He aspired to attach the Ottoman forces permanently to his interests, and hoped to achieve this by giving his second daughter Theodora in marriage to their ruler, despite differences of creed and the disparity of age. However, in Byzantine and in Western European history, dynastic marriages were quite usual and there are many examples which were much more strange.
The splendour of the wedding between Orhan and Theodora at Salymbria (Silivri), is elaborately described by Byzantine writers. In the following year, Orhan with Theodora visited his imperial father-in-law at Uskudar, (then Chrysopolis) the suburb of Constantinople on the Asiatic side of the Bosporus where there was display of festive splendor. However, this close relationship was soured when Byzantines suffered from marauding migrant Turcoman bands that had crossed the Marmara Sea and Dardanelles and pillaged several towns in Thrace. After a series of such raids, Byzantines had to use superior forces to deal with them.
During Orhan's reign as the Emir of Ottoman Principalty, the Byzantine Empire was on its last stages of its failure. The greedy ambitions of Italian maritime states and the aggression of the Turcomans and other city Turks had its effects on Byzantine Empire but that empire was in open conflict with itself for some time as well.
During these years the Byzantine Empire was so weak that the competition for commercial supremacy in the surrounding seas around it became a bone of contention for the Italian maritime commercial city states. The Republic of Genoa possessed Galata, a separate Genoese city opposite the Constantinople across the estuary of Golden Horn which provided the main harbour for the great city. The Genoese had fought and defeated the Byzantines earlier in 1348 when the Byzantines had decreased their customs tariffs in order to attract trade to the Byzantine side of the Golden Horn. In 1352 the rivalry for trade led to a war between Genoa and Venice. Genoese, in trying to repel a Venetian fleet from destroying their ships in Golden Horn, bombarded the sea walls of Constantinople and pushed Byzantines to ally with Venetians. Venetians gathered together a large naval force, including hired fleets from King of Aragon and from Byzantine Empire of John VI Cantacuzene. Unfortunately for Venice and its allies the sea battle between Venetian fleet under the command of Nicolo Pisani and Genoese fleet under Paganino Doria led to defeat of Venetians and Byzantine allies . Orhan was against the Venetians, whose fleets and piratical raids were disrupting his seaward provinces, and who had met his diplomatic overtures with contempt. The Venetians were allies of Cantacuzene; so Orhan sent an auxiliary force across the straits to Galata, which there co-operated with the Genoese.
In the midst of the distress and confusion with which the Byzantine Empire was now suffering from, Orhan's eldest son, Suleyman Pasha, captured the Castle of Tzympe (Cinbi) in a bold move (1356), which gave the Turks a permanent foothold on the European side of the Dardannelles Straits and started to settle migrant Turcomans and town-dwelling Turks in the strategic city and castle of Gelibolu (Gallipoli) which was devastated by a severe earthquake and which was totally evacuated by its inhabitants. Various financial inducements offered by the Byzantine emperor John VI Cantacuzene to empty the castle and the city were refused by Suleyman Pasha and the emperor's pleas to his son-in-law Orhan to meet personally and to discuss this was either refused or could not be accomplished due to ill-health at old age of Orhan.
This military situation was not resolved, in part because the eruption of the hostilities between John VI. Cantacuzene and his co-emperor and son-in-law John V. Palaeologus. John V. was dismissed from his imperial post and exiled to Tenedos; Cantacuzene's son Matthew Kantakouzenos was crowned as the co-emperor. But very soon John V. Paleologos was able to return from exile with Venetian help and to conduct a coup taking over the government of Constantinopolis. Although the two men came to agrement to share power, John VI. Cantacuzene resigned from his imperial post and became a monk. Each of these two contestants for power was continually soliciting Orhan's aid against the other, and obtaining that aid according to what seemed best for the interests of the Ottoman sovereign.
Orhan was the longest living and the longest reigning of the future Ottoman Sultans. In his last years he had left the state powers in the hands of his second son Murad and lived a secluded life in Bursa.
In 1356 a very unusual event has occurred. Khalil, the son of Orhan and Theodora, was being taken round the Bay of Izmit on a boat. A Geneose commercial boat captain, which was conducting piratical acts alongside commerce, was able to capture the young prince and take him over to Phocea on the Aegean Sea, which was under Genoese rule. Orhan was very much upset by this kidnapping and conducted talks with his brother-in-law and now single Byzantine Emperor John V Palaeologos. As to the agreement, John V with a Byzantine naval fleet went to Phocea; paid the ransom demanded of 100.000 Byzantine gold pieces and brought the young boy Khalil (who was his nephew) back to Ottoman realms.
In last years of life of Orhan, (in 1357) his first son, heir and the most experienced soldier and statesman Suleyman Pasha died at a hunt on European side of Orhan's realms. Orhan was very much affected by it
Orhan died soon after at 1359 at the age of seventy-five, after a reign of thirty-three years. He is buried in the tomb of his father Osman, called Gumuslu Kumbet in Bursa.
During his reign, some of the most important civil and military institutions of his state were founded, in the Western provinces of Anatolia, but was also planted on the European continent.
Orhan, as allowed by Muslim religion, had several concurrent wives and concubines. The order of his marriages in not certain.