I have found that using the large oak leaves like that does not allow enough water to reach the ground (plants/tree roots, etc). If you can shred/crush them with a leaf shredder or lawn mower this will work better.

Deep layers of Oak leaves can smother a lot of perennials too. A thin 3-4" layer works well and even allows some kinds of plants like Rudbekias to send out runners and spread.

depends on your oak. I have willow oak, with the little thin, narrow leaves, and as long as I don't pile them too deep, they make great mulch and weed-blocker. also compost beautifully. otherwise, i agree, shred 'em. you can also burn 'em off to get lovely fine ash for lime. Just do it very carefully because otherwise they will blow sparks all over the place.