It becomes easier when you consider the terms independently and in a non-pejorative manner. Left-wing and right-wing really infer an approach towards modernity (e.g., secular vs religious, republican vs monarchial, internationalist verus nationalist etc). That can be quite independent of the degree of totalitarianism used (and these things must be studied by degrees). The more extreme expression of collectivism, where the state replaces the will of the people, is indeed fascist which, whilst always totalitarian, also tends towards authoritarianism as well. So... not really an oxymoron at all.