Solaris was mostly open source; there are plenty of derivatives. Look up "illumos". I use Joyent's SmartOS pretty regularly and it's pretty solid.

EDIT: unless you meant the video game, which someone just told me is a thing… oops.

I meant the OS. Afaik,opensolaris was open source but solaris itself had closed source components,illumos and the rest ate forks not maintained and supported versions of solaris

Oracle effectively started to close source Solaris with 10, I think and then completely closed sourced it with 11. Illumos is definitely a fork, but given that Oracle effectively doesn't work on Solaris anymore it's more or less picked up where Solaris left off, it's probably superior to the commercial Oracle version at this point.