You may have answered your own question:-? or just about. Dictionary.com has an interesting usage note on the word 'redundancy' which includes:
'..the use of what is regarded as an unnecessary modifier or qualifier can sometimes be justified on the grounds that it in fact makes a semantic contribution. Thus a hollow tube can be distinguished from one that has been blocked up with deposits...'

This applies to your 'happy grin'. For it could be a mischievous grin, couldn't it.

But you might like self-reflexive usages like a superfluous redundancy, or a redundant tautology:-). But you could glean some other opposites for oxymoron, depending on how exactly you want to define your oxymoron (literally 'pointedly foolish'), but the obvious answers seem fine for 'positive optimists' and the like.