If you want strong minions you have to sacrifice some health stacking and some nice abilities. So oak is gonna be relatively fragile. If you gank him or just attack him (spit for live) he has to shield, get out or die. he can annoy you with his minions but if you or one of your allies get bisshops he probably needs to attack you directly to kill you or push you away.
You can prevent minions from hitting you by getting abilities like ooze, fire nova (basic AOE), or mass charm (Massive stuns)
Minions also have another problem, they are nice early on for lane control and killing towers but in lategame they arent that useful and I'd much rather have penitence and shield in lategame then to be able to do insane damage to my enemies since then they will have 6k health and 40% armor. (possibly even more)
There are many demigods that you can easily defeat by not getting raped by them untill lategame. (this counts especially on cataract) Reg, tower Rook, Queen, full minion oak, full minion ereb all need to win before lategame when ppl are porting around capturing and defending portals and can afford to go back to the health crystal more often. for lategame you need, locks, ports and sigils, interupts, durability and ways to escape. (bat swarm and shield for exampel) many of the demigods above dont have that and lose because of it. If oak goes full minion he will probably have to sacrifice at least some of the lategame-neccesities I named.
