The SmartOS and OmniOS folks work together from time to time. For example, I believe Bhyve was first ported from FreeBSD to OmniOS before it eventually came to SmartOS.
Other way round. Like LX, into SmartOS first, then brought across into OmniOSce. But yes, OmniOSce picks up a lot of SmartOS features, whereas some of the other distros are vanilla illumos.