OnCommand won't be aware you've enabled network compression, either at the volume or qtree level. If ONTAP supports qtree level compression, OC won't do anything to stop it.
That's not entirely true. There are cases when OnCommand will re-write the snapmirror.conf entry and can in the process, disable compression. The case I know of is if you set the hostPreferredAddr1 or hostPreferredAddr2 attributes on a storage system. When you do that, OC needs to add a connection entry for the affected SnapMirror relationships and that can disable compression as a side effect.