How does oak barrel work exactly?

Hi guys, i got a question.

I just started my brawler a few days ago (right now level 35) and i was wondering...Which Brawler skill should i put off so i can max Oak Barrel? I was thinking one of these three.

- Corkscrew Punch lvl20
- Elbow lvl20
- Double Uppercut lvl20

Which one of these skill become replaced/useless in 3rd or 4th jobs?

None, you just don't max mp recovery and max barrel instead, then at 11x, you don't get shockwave to 21 and get mp recovery to 10 instead.