Mulligans
General Mulligans
You wanna keep every card that ramps, in every matchup, since your curve is high. Barnes is also always a keep, since it has high-roll potential. Oakheart is a greedy keep, but it's pretty much an instant-win once you play it, so if you think you can keep him, you should.
Aggro Mulligans
Aggro decks go wide these days, so these 2 cards can help you at lot.
Control Mulligans
Against control, you just want to ramp as fast as possible, since control decks are slow, you can be greedy and keep master oakheart and dragonhatcher, although only if the rest of your mulligans are good. Naturalize helps against voidlords.Another idea I wanted to work out. It’s a high-roll deck so I wouldn’t play it in ranked.
The idea is that Master Oakheart summons Drakkari Enchanter, Dragonhatcher and a random minion with 3 attack. Barnes has potential to summon a Dragonhatcher, Ysera, The Lich King or Deathwing, Dragonlord as well, although a good outcome isn’t guaranteed. Since you want to ramp as fast as possible, all cards that aren’t big are ramp and control cards.
Drakkari Enchanter is also a good topdeck if you have developed a board, since it works with Dragonhatcher, Ysera and The Lich King.
I think 1 copy of Ultimate Infestation is enough, since the curve of the deck is very high.
Edit: just noticed this site has a very similar list, so you can treat this list as an alternative or as a guide.