Here is some really useful info if you are stopping overnight at OTP...
Bucharest (OTP) lounge recommendations?
A review of the Mastercard Lounge (used by BA)...
Review of Mastercard Business Lounge at OTP (Bucharest)
BA Lounge Database (search for OTP)...
BA Lounge Database | Europe
If that is correct then it looks like the lounges are 24hrs