Should any fallen branches of Oak work work, if I soak it first ?
Yes, any part of the oak tree will work.
Remove the bark first, and make sure it's not rotted/soft. Then sterilize it either by boiling (if it fits in a pot) or by soaking in a 1:20 bleach:water mix for a day then soaking it in water for a week, changing the water every other day, to get the bleach out.
Boiling is viewed as safer for the fish, it's also stinky. These pieces were too big for a pot so i had to go the bleach route (in a rubbermaid 25-gal bin, outdoors). I've done this before and never had bleach poisoning/gill issues, the chlorine in the bleach really wants to be a gas and escapes at the first chance. That's why bleach smells so bad.