Ultimeh and I played a 1v1 Sedna vs Oak on Prison, the match was a complete joke I was able to keep all three center flags playing with basically one hand.

Spirits need a swift nerfing, I feel there are a few options:

-Diminish spirit damage as they move farther from Oak (something like:  damage*[20m/distance], with a maximum multiplier of 1.0)

-Just flat out nerf them in general, this may leave Oak lacking in the balance department.

-Lower their HP to incredibly low amounts.

I like option #1 however this still leaves Oak a murdering machine at close range, Sedna would be more than capable of fighting it if they got a medium damage nerf.

I played with both of them a little aswell and my thoughs are quite similar to yours:

1) oaks spirit are overpowered, my solution would be to reduce their hp and have 8 max

2) sedna is an "healer" so not having great damage is quite fine, however the yeti i agree do very little damage and are kind of useless for the mana cost, i'd increase their attack speed and maybe give them a very very little aoe to damage everybody nearby them.

I agree with tesb, bringing other minions to the level of oaks spirits would be a better idea (with maybe a slight nerf to the general level of spirits). At the moment, oaks spirits are the only ones worth summoning; if you nerf them then we'll just see no generals using their minions. If all minions are brought up to that level, it'll actually become viable to play one of the other generals as a general rather than assassin, and will act as a partial counter to oak's minions (you can set your minions fighting oaks).