[quote=@LokiLeo789]
In my collab, I slowed down the molecules in the air around my opponent, would that encase him in ice and crystal and trap him? Is that possible? Is that OP?
[/quote]

Dude first off, that is going way overboard especially for a student whose only beginning to use his semblance in combat situations. 

Secondly slowing down the air around someone would effectively just make the air thicker. It would make it harder to move but not impossible to fight. 

In another RP I'm involved in I have a character who has telekinesis and at his most advanced stages of mutation he is able to condense air molecules to form a shield. If we assume the same physics apply between the two then this makes the shield ineffective against energy weapons because they can use the air as a medium to travel AKA dust will get through the barrier. You are effectively constricting something whose very base nature demands that it spread and move as far and as freely as it can. Taking into account that the semblance is fighting the natural properties of air this would require an intense amount of energy like say more energy than someone whose in the middle of combat can adequately produce without passing out.

My personal opinion and I think Lug's as well is that you shouldn't be able to do this [i]but[/i] in the interest of not wanting to hamper creativity if you do wish to keep this as a facade of your semblance then you need to add some major side effects.