#knowledge pool

It depends on whose turn it is.

Whenever a player casts a spell from their hand, both Possibility Storm and Knowledge Pool will trigger, but the order of those triggers depends on whose turn it is.

If it’s your turn:

 Your Knowledge Pool’s trigger goes on the stack first, with your opponent’s Possibility Storm’s trigger above it on the stack. 

Possibility Storm’s trigger resolves first, exiling the spell and other cards until a card of the same type is revealed and cast (or not). All cards revealed this way that weren’t cast (including the original spell) are put on the bottom of its owner’s library in a random order. 

The spell cast from Possibility Storm’s trigger resolves next. 

Knowledge Pool’s trigger goes to resolve but does nothing. The spell that triggered Knowledge Pool is no longer on the stack, so it can’t be exiled and thus the trigger has no effect.

If it’s your opponent’s turn:

Your opponent’s Possibility Storm trigger goes on the stack first, with your Knowledge Pool trigger on the stack above it. 

Knowledge Pool’s trigger resolves first, exiles the spell, and allows the spell’s controller to cast another nonland card exiled by Knowledge Pool. 

The spell cast off of Knowledge Pool resolves next.

Possibility Storm’s trigger resolves. The spell that triggered it is no longer on the stack, so it can’t be exiled, but that doesn’t matter since doing so isn’t required for Possibility Storm to have its full effect, unlike Knowledge Pool. The game remembers how the original spell looked as it last existed on the stack and uses that information when revealing cards and looking for a new spell to cast (or not.) All cards revealed this way that aren’t cast are put on the bottom of their owner’s library in a random order.

Finally, the spell cast of Possibility Storm resolves.

Knowledge Pool only applies to spells a player casts from their own hand. Casting a spell from another player’s hand doesn’t match this criterion and won’t trigger Knowledge Pool’s ability.

Knowledge Pool

When Teferi and Knowledge Pool are both on the battlefield, then the opponents of Teferi’s controller won’t be able to resolve any spells cast from their hands. Whenever they cast a spell from their hand, Knowledge Pool’s second ability will trigger and exile that spell. The player won’t then be able to cast one of the exiled cards off of Knowledge Pool because they can’t cast spells at that time thanks to Teferi’s last ability.

Teferi and Omen Machine isn’t a hard lock in the same way, but it can basically stop your opponents from getting any new non-land cards from their libraries since they won’t be able to cast any non-land cards they reveal to the Omen Machine trigger. They’ll still be able to cast other spells from their hands and the like, though.

Ok, so...

When Knowledge Pool enters the battlefield, each player exiles the top 3 cards of their library under knowledge pool. Only the nonland cards really matter, but the lands are still exiled.

When a player casts a spell from their hand, Knowledge Pool’s second ability triggers. When the trigger resolves, if that card is still on the stack, Knowledge Pool exiles it and then allows that spell’s controller to cast any other nonland card already exiled by Knowledge Pool. The card that was originally cast is now exiled with Knowledge Pool and can be chosen for the next spell cast from hand.

The type of the card being cast doesn’t matter since Knowledge Pool grants permission to cast the spell when the trigger is resolving. You can cast an instant into Knowledge Pool and get a creature out of it, for example.

Nothing.

Knowledge Pool only interacts with cards cast from a player’s hand. Cards cast from the graveyard or exile resolve unimpeded.