Hello there.

If I got Ojutai Exemplars on the battlefield and I cast End Hostilities, do the Exemplars return before or after the mass removal?

Before. The entire ability resolves before the removal would resolve. So it would be exiled, then returned to the battlefield tapped, then the removal would hit.

I was afraid it's like this... otherwise it would be to powerfull I suppose.

Thank's a lot, although you gave the "wrong" answer ;)