大体(だいたい)Most・mostly, Adequately・generally, For the most part, Roughly・approximately, In the first place

Both can mean 'most', as in だいたいの人／おおよその人 'most people'. They can also both mean 'approximately', e.g. だいたい３時間／おおよそ３時間 'about three hours'. However, おおよそ is much more formal and mainly used in writing. だいたい can also express 'in the first place', e.g. だいたいなんでいるの？ ('Why are you here in the first place?)' or 'roughly/adequately', e.g. だいたいでいいから ('Roughly is fine.), while おおよそ cannot.

These can be used interchangeably to express roughly or about. だいたい is more casual and common in daily speech, as in だいたい百人来た (About a hundred people came). おおよそ sounds slightly more formal or written, as in おおよそ見当がつく (I can roughly guess). Note that だいたい can also appear as a negative interjection, as in だいたい何でいるの？ (Why are you here in the first place?), while おおよそ cannot be used this way.3

Both だいたい and ほとんど can express 'most' or 'almost all,' but they also have distinct uses. For example, だいたいの人 and ほとんどの人 both mean 'most people', and だいたい一緒 and ほとんど一緒 both mean 'almost the same'. However, they diverge when ほとんど is used negatively as in ほとんどない 'almost none', as well as when だいたい means 'generally' or 'approximately' (だいたい５時に起きる), as ほとんど cannot be used here.