Officially, it's singular. The verb is always singular. You always say *on est* ; *on sont* would be incomprehensible.
But since *on* is more and more replacing *nous* in spoken French, you often find now an inconsistent usage: *On est allés* (we went) with a singular verb but a plural participle. The correct form should be *on est allé*, but many good writers and journalists use the plural in the participle if the meaning is "we" and not "one" or "they". In a sentence like *on a raison de dire*, *on* means "people", "they", "one" : "people are right in saying", "it is right to say". But when after a soccer match you here supporters chanting *On a gagné*, it definitely means "we won".
But since *on* is more and more replacing *nous* in spoken French, you often find now an inconsistent usage: *On est allés* (we went) with a singular verb but a plural participle. The correct form should be *on est allé*, but many good writers and journalists use the plural in the participle if the meaning is "we" and not "one" or "they". In a sentence like *on a raison de dire*, *on* means "people", "they", "one" : "people are right in saying", "it is right to say". But when after a soccer match you here supporters chanting *On a gagné*, it definitely means "we won".
