Can a hydroxyl group be called a carbonyl group?
No, a hydroxyl group is different than a carbonyl group. A hydroxyl group is an O-H group, while a carbonyl group is a C=O (double bond) group. Perhaps you are thinking of a carboxyl group, which is a sort of hybrid of the 2 groups. Carboxyl groups are C-O-O-H, essentially a merge of the hydroxyl and carbonyl groups.