A 2" oak slab will be really heavy and your swinging motion will be
sluggish, at best. Test: At arm's length, swing an empty and loaded
suit case and see/feel the difference. If you are to make your own
swing, I'd suggest cypress, Eastern Red cedar or Redwood and don't put
any finish on it. Seat and backrest made of 3/8" - 1/2" thick slats,
with gaps (#6 finishing nail diameter) between, so that it dries fast
after a rain.

If you try to paint or finish any outdoor woodwork, in hopes it will
be sealed, you're thinking wishfully. You are unlikely to seal every
little nook & cranny and water will seep/wick under the coating and
the wood will rot, even cypress, cedar and redwood. Not so much the
wetness will casue the rot, but the decay from dirt, grime,
bacterior, and other abnormal no-see-ums/growth associated with damp
warm environments..... add spilled warmed beer in the mix, also!

1/4" chain, as any smaller may wear out and break faster than you
think, depending on the connections you use ---- Some hardware stores
have eye bolts with pressed-in high density plastic rotating bushings,
specifically for hanging porch type swings. Small size S hooks hooked
on eye bolts, alone (no plastic bushing), will wear thin and break.

Sonny