I don't disagree with any of the opinions here regarding tank size or the possibility that the O will try to eat the BN, which could be deadly for both fish, I did do this for a couple of years with two Os in a 210g tank with a BN. One O was 12"+, the other a juvenile.
If you insist on doing it, the key is to have many hiding places, like crags between rocks, for the BN that the O cannot access. My BN stayed in these crags during the day and came out at night when the O was sleeping. The BN is nocturnal anyway so that's not really unusual for it. Just recently I've had to remove my O's to hospital tanks so the situation has ended for me at this point.