When outsourcing, never assume that the company or developper will do all the job for you. You need to pay attention to what is done of a daily basis.
I would also strongly recommend having an agreement on project governance from the beginning. It’s important to understand how the outsource team is going to interact with the core team and you do not want to discover this on the way.
Enforce quick release cycles too so you can better track how things are evolving.
One last point: do not underestimate the effect of time difference if you are working with developper living in a different one.