A polyfunctional phenylene ether oligomer (B) having 3 to less than 9 phenolic hydroxyl groups, obtained by reacting a polyfunctional phenol (A) having 3 to less than 9 phenolic hydroxyl groups per molecule and having alkyl groups and/or alkylene groups at 2- and 6-positions of at least one of the phenolic hydroxyl groups with a monohydric phenol compound of the formula (1), derivatives thereof, resin compositions containing the derivatives, curable films obtained therefrom and cured films thereof, ##STR00001## wherein R.sub.1 and R.sub.2 are the same or different and represent a hydrogen atom, a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group, and R.sub.3 and R.sub.4 are the same or different and represent a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group.