Evans' first invention was in 1777, when he designed a machine for making card teeth for carding wool. He went into business with his brothers and produced a number of improvements in the textile industry. His most important invention was an automated grist mill which operated continuously through the use of bulk material handling devices including bucket elevators, conveyor belts, and Archimedean screws. Evans described this invention in The Young Mill-wright and Millers' Guide. He patented this invention in a few states and, when the US patent system was established, in the federal patent system. Evans devoted a great deal of his time to patents, patent extensions, and enforcement of his patents.
In 1792 he moved to Philadelphia, Pennsylvania.
He produced an improved high-pressure steam engine — his second most important invention. For some years he contemplated the idea of applying steam power to wagons. He was granted a patent for a steam-carriage design in 1789, but did not produce a working example of such a machine until over a decade later. (See below on his Oruktor Amphibolos). Part of his difficulties was a failure to get financial backing. After lack of support in his native land, in 1794 he sent copies of some his designs to Great Britain in an attempt to interest investors there.
As Evans designed a refrigeration machine which ran on vapor in 1805, he is often called the inventor of the refrigerator, although he never built one. (His design was modified by Jacob Perkins, who obtained the first patent for a refrigerating machine in 1834).
The device for which Oliver Evans is best-known today is his Oruktor Amphibolos, or "Amphibious Digger", built on commission from the Philadelphia Board of Health. The Board was concerned with the problem of dredging and cleaning the city's dockyards, and in 1805 Evans convinced them to contract with him for a steam-powered dredge.
No technical drawings of the device survive, and the only evidence for its design come from Oliver Evans' own descriptions, which are contradictory, and most likely exaggerated. He describes the vehicle as a long 15 ton scow, powered by a 5 horse-power steam engine. For a demonstration of his long-held beliefs in the possibility of land steam demonstration, Evans mounted the hull on 4 wheels and may have connected the engine to them, to drive it through Philadelphia streets on the way to the river. The small size of the engine, the large size of the vehicle, and the lack of any contermporary evidence other than Evans' own writings for it make this seem unlikely. Evans claimed that his dredge was the first self-powered amphibious vehicle, as well as the first self-powered land vehicle in the United States (steam powered road vehicles had already been used earlier in France and Great Britain). The Oruktor Amphibolos was never a success as a dredge, and after a few years of sitting at the dock was sold for parts.
Subsequently, Evans wrote about the Oruktor in many publications, and each time the achievements became more impressive. He also 'corrected' the date of the machine from 1805 to 1804, possibly in a dispute about steamboat patents, and this inaccuracy has since been perpetuated by several commentators.
Oliver Evans wrote up proposals to mechanize road vehicles, but failed to get backing from investors, who saw the scheme as impractical. In 1812 he published a visionary description of the nation connected by a network of railroad lines with transportation by swift steam locomotives. It should be remembered that at the time the locomotive was little more than a crude curiosity, and no attempts to use it for long distance transport had yet been made; see: History of rail transport.