In 1830 Oastler met John Wood, a worsted manufacturer from Bradford who agonised over the need to employ children in his factory. Already an abolitionist, Oastler, after a lengthy meeting with Wood, decided to join the struggle for factory legislation, and wrote a letter on the subject of "Yorkshire Slavery" to the Leeds Mercury newspaper.
From this time forward, Oastler dedicated himself to the battle of what was now known as the 10-Hour Movement, and to campaigning for Poor Law reform.
By 1836 Oastler was urging workers to use strikes and sabotage. This proved his downfall. Thornhill, hearing of his speeches, sacked him as his steward in May 1838 and called in unpaid debts. Oastler was unable to pay up and was jailed for debt on 9 December 1840.
It took his friends and supporters more than three years to raise the money necessary to pay the so-called Factory King's debt. 'Oastler Committees' were formed in Manchester and other places to assist him, and 'Oastler Festivals' - the proceeds of which were forwarded to him - were arranged by working men. In 1842 an 'Oastler Liberation Fund' was started, and by the end of 1843 the fund amounted to £2,500. Some of Oastler's friends guaranteed the remaining sum needed to gain his release, and in February 1844 he emerged from the Fleet Prison.
Having maintained his campaigning activity while in prison by publishing weekly Fleet Papers devoted to the discussion of factory and poor-law questions, Oastler continued his work and achieved some sort of success when the 1847 Factory Act restricted children to a 10-hour day in cotton mills. But it was not until six years after his death in 1861 that the act was widened to encompass children working in all factories.
His later years involved editing a weekly newspaper called The Home, which he commenced on 3 May 1851, and discontinued in June 1855. He died in Harrogate in 1861 and was buried in Kirkstall churchyard.
A public house (previously a chapel) in Park Street Market, Brighouse, and a school in the Armley district of Leeds, West Yorkshire, also bear his name.