On one ticket that's tight, so it's very much at your own risk on two tickets. That said your Delta flight could be up to about 30 minutes late and you may still make it. Just get your boarding pass in advance via OLCI and proceed to Flight Connections. There is a BA desk just in front if needed but if you have boarding passes there's no need to do anything there. Note that if you want to change the flight to a later one, you can normally do this online on the BA App, and to some extent you can be guided by any delays in ATL, since once you are after midnight UK time then you can decide whether Delta is on time or not, particularly if you get wifi on flight. In that respect don't do OLCI until you know the score, since Free Change on the Day generally works best if you have not OLCI.