I'm not a pro, though i usually play Oak, mostly assassin. Looking at your replay, a few notes:
1) You kept both your monks for yourself. Due to the way monks work, two wouldn't heal you any more than one, so might as well give one to one of your mates, usually one who does more melee fighting (UB in case of that game).
2) At around 4:00, you went for the XP flag instead of defending the tower. One demigod is sometimes enough to chase two away from a tower, and you going there may have encouraged the TB to stick around too (making it 2v2 at your tower). XP flag is important, but you can cap it later; can't rebuild the tower, and losing it is a big strategic disadvantage.
3) The first to equipment items you got were Vlemish Faceguard and Plenor Battlecrown. That's 3,250 gold. For that price, I'd get Banded Armor (550) and Scaled Helm (550), then get Currency 1 upgrade for the citadel (1800) which will put you ahead in gold later. You'll end up with more HP and HP regen => end up staying in field longer. True, less mana, that means though you have to use your abilities carefully.
4) Paid attention to what's happening on the other side of the map and TPed in - good.
5) Running many times past enemy-owned flags and towers - bad, invites ganks, good thing they didn't react that fast.
6) 1v1 vs Sedna with monks and more HP than you at 8:20 - that's a losing battle, can't win.
7) No one teleported in to save the portal at around 12:30 (? time approximate).
8) Past 6:00 or so it's best to always carry at least one flag lock. If you had a flag lock, you could have prevented the opponents from retaking your portal at 14:00.
At that point the opponents have enough advantage over you, so you couldn't have done much.
Aside from that, your skill and favor choice w/ oak is pretty much the same as mine, except I also try to carry a Sigil of Vitality, to help me stay and fight longer (or help me escape).