J/w which matchups do we use it and when do we not?

Yes

Almost always.

Most important vs:
Bowa
Vt/smite/foh
Necro
Hammerdin
Barb
Blizz

Ele dru pierce it, but it may make him miss with volcano if u are swirled, and there is less chance his summons will trigger a fhr/fbr animation on you.
Light sorc pierce it, but if they use high level Thunderbolt you should use.

Oak may make fb sorc hit you easier due to ur making urself a bigger target and fb is splash damage.

Chars like rabies etc may use ur summons to infect you with rabies, but after you are infected oak should be up to keep hp as high as possible, same for ow and other psn attacks.

Ty!