 He first appeared in London in the late 1580s, as a fencing teacher in the entourage of Sir Florence MacCarthy, chief of the MacCarthy clan, (the MacCarthy Mór),
whose own loyalty to the Crown was deeply suspect, and who was later accused by his enemies of being a party to O'Collun's plot.About 1590 O'Collun went to the Spanish Netherlands, where he entered the service of the renegade English Roman Catholic soldier Sir William Stanley