The Oxford definition gives the option to assign both, positive and negative traits to every possible race. Racism can work both ways this way (as my teacher said to me, you are a positive-racist!) after assigning the most positive traits I could find to people from random nations and that is because it’s not universally used to belittle other races as inferior or the own race as universally superior (which is the usual use-case, but not the only one possible).
The UN definition does not leave this option. According to the UN, racism is only negative and can only be positive to the one that is trying to establish dominance which makes it negative again (by assigning the most positive traits to the dominant race is bad because it makes this race the dominant one that tries to achieve dominance). I.e. everyone is worse than X.
Also, social groups ← which aren’t even included in the oxford definition. Thereby expanding the definition to pretty much everyone, since everyone can have a social group that is targeted by harassment / violent action. Having social groups in there essentially says: everyone is racist.
With todays social acceptance, I could claim that, according to the UN, my gender is the one of the “Fedora-wearing-vaping-douchebag-that-clouds-your-favorite-sushibar-with-funny-smells-and-ruins-your-meal-thisway”, because that’s what I could do and I would be a minority in an instant and could rightfully play the racist card against you if you would dare to critize me, all thanks to “Social groups”.
Oxford doesn’t give you that option, which is a good thing.
You don’t consider that a vast difference?