No, they are not becoming partners with OMG.
Julian from GNT explained:
https://www.reddit.com/user/julian_z"imapp (company where Golem was started) is contractor to OmiseGO and will work on the project with them.
Golem devs will not work on OMG and OMG devs will not work on Golem. Both teams will be in the same building, but most likely on different floors."