The tribunal continued the case to decide on compensation.
Peter Duncan, the college's principal, has already undertaken to consider reinstatement as one of a number of "parallel tracking activities", although the college board has not yet ruled out an appeal against the tribunal's judgment (TESS, April 2).
Mr Duncan is pledged to restore "constructive industrial relations" at the college.
The tribunal had already taken the unusual step at the end of last year of issuing its findings through an informal statement by Hugh Murphy, its chairman, who described Mr O'Donovan's offence as "utterly trivial".
He was fired over remarks made in a corridor meeting with a member of the college's board of management which the college described as "intimidatory, threatening and confrontational".
The college denied that Mr O'Donovan, president of the Educational Institute of Scotland's Further Education Lecturers' Association, had been sacked for his union activities.
But the tribunal, in its written judgment issued on Monday, said it was "satisfied that the underlying cause of that dismissal was the fact that the applicant had proved himself such a nuisance in the course of trade union activities".
The tribunal suggested in particular that Mr O'Donovan's opposition to a reorganisation of posts at the college had clouded the management's judgment of him to such an extent that he was dismissed.
The written decision said that the college had exaggerated its evidence against Mr O'Donovan and made "mountains out of molehills (if there were ever molehills in the first place)".