Click on the tab titled "slip, slide, and collide," then the link at the bottom titled "see what happens at different plate boundaries." what happens when two oceanic plates collide? what negative impacts for human populations can result from this type of action?

When two oceanic plates collide, the older oceanic plate will subduct under the younger oceanic plate and a trench and/or a volcano will form.
The process could create earthquakes that lead to the formation of Tsunami. If the scale is large enough, the tsunami could reach populated area and destroy everything it passed.