To be completely honest, most people who complain about Oak's spirits are the kind of people who like to stand still and expect to "tank" them. Currently spirits hit for roughly 80 damage each with maxed Morale, and have about 2500 HP with a couple minion buffing items.
Movement is an integral part of a game like this, and if you are not capitalizing on the fact that they do NOT hit moving targets with normal attack commands it is a player problem. The only way to hit someone moving with Oak's spirits currently is to move said spirits ahead of your target, let them auto attack, and repeat the process. There are a number of problems with this, though. First, if any mobs are around more than likely the spirits will attack them. Second, when you use a "Stop Movement" command, the spirits tend to spread out like fighter planes in an RTS would *Cough*. Third, this may be considered "Advanced Tactics" but all that would be is an excuse meant to overshadow the main problem.
I propose that the spirits be lowered to about 1700 HP WITH minion buffing items, and their damage kept the same. In addition to this, they should move at the same speed as Oak (Counting +Speed items) and also be restricted from flight. With this in mind, I'd assume their AI would be fixed so that they can hit moving targets. It would take two AOE spells to drop all the spirits, or one Rook hammer. You need to bear in mind that you cannot nerf their HP too much as competitive play will most likely take place in the form of 4v4 or 5v5. 5v5 being the prefered format for all competitive leagues, but I doubt this because currently duplicate Demigods on the same team is inbalanced. In a game with this many players the AOE available will neuter Oak.
EDIT: I've given it some thought, and I'm hesitant suggesting nerfing their HP. In a 4v4 or 5v5 all it would take is a couple Demigods combining their AOE to kill them. The main problem people have with them (I have with them) is that they can cover ground too quickly and seem to avoid certain AOE abilities due to them flying.
In my opinion (And other "advanced" players I know in beta) this would solve the current Oak spirit problems.
I plan to do a full write up on Oak soon, as this is only the surface of the problems he currently has.