William Osborn(e) appears to have enlisted in the British Army about 1755. He was about 32 years old making him a rather old recruit. It isn't clear from available sources if he enlisted directly into the Royal Irish Regiment or if he was drafted (transferred to) the 18th Foot at some time prior to 1767. In that year, He was listed as being from Westfall, but no such places existed. He was most likely from Westphaltown, about 12 miles from Dublin. He embarked for America from Cork Harbor in May 1767 arriving at Philadelphia in July. He was assigned as a private in Captain Hugh Antrobus's Coy which become Benjamin Johnson's Company after Antrobus's death on the ocean voyage to America. With Johnson's Coy, he was ordered to Fort Pitt where he remained from the summer of 1768 until the post was mostly abandoned in the summer of 1772. Johnson's Coy returned to the North Barracks at Philadelphia where it remained for the next two years. In June of 1773, Osborne was transferred to the General's Coy. This may be the direct result of his being selected by the chaplain as a servant. He remained as the chaplain's servant through his service in Boston. In October 1774, that company was one of five marched through New Jersey to NYC where that detachment replaced the 23rd Foot which had been ordered to Boston. Osborne was most likely involved in the stand off against the NYC Sons of Liberty on June 6, 1775, when the remaining troops of the Royal Irish evacuated from the NYC Barracks and took shelter aboard the HMS Asia. Osborne and his mates arrived in Boston shortly before Bunker Hill, but they observed the battle and didn't participate.
Osborne was transferred to the Lt. Colonel's Coy of the 5th Foot at Boston in December 1775 when the men of the 18th were drafted into the other regiments remaining in Boston. Osborne was later transferred to Captain Baker's Company in the 5th Foot by 1777. He was listed as sick on the returns of 23 April 1777 and 8 March 1778. It isn't clear but appears unlikely he had been wounded. He was listed as discharged and recommended from the 5th Foot on 5 May 1778, but he examined at Chelsea on 13 May 1778. Most likely he had been embarked for England earlier in the spring and simply paid through 5 May 1778. According to the Chelsea records, he was "worn out" from 23 years of service. He listed his trade as a bucklemaker prior to enlisting.