watching olly play defense shows me, as pointed out above, that his man to man defense will never be particularly good. he simply does not have good lateral movement or foot speed. that is why he plays off his man so far.
compounding this is him playing center, that is, the final defense against drives. he simply cannot match up well against faster players.
i would imagine though, that if he were to play pf and there was a credible to good defender at center, olly's defects would be far less glaring.