`what happens when two plates carrying oceanic crust collide

Subduction happens.
When two plates carrying oceanic crust collide, the denser plate subducts below the other at deep ocean trench. Subduction happens as the oceanic plate sinks down the continental plate. On the other hand, when two plates carrying continental crust collide, the said process (subduction) does not happen. It is because, neither piece of crust is dense enough to go down very far into the mantle. Instead of subduction, the collision squeezes the crust into great mountains and mountain ranges.