Irving was jailed in Austria for his Holocaust denials and Griffin's party is anti-immigration, anti-Muslim.
University unions President Luke Tryl, denied claims the forum was simply about garnering publicity.
"I find the views of the BNP and David Irving awful and abhorrent but my members agreed that the best way to beat extremism is through debate," he said.
Oxford Member of Parliament Evan Harris, a Liberal Democrat, also a part of the debate, said protests weren't an effective way to deal with the controversy, the Oxford Mail reported.
"I don't want a bunch of students telling other students or my constituents or me who I can talk to within the law because that way lies effective despotism," Harris said.