OIDs are not stored in the database, unless you are using the Universal Device poller in which those are stored in the Custom Pollers tables.
To see the OIDs that are used, they are listed in the Success Center. CPU and memory utilization Object IDs (OIDs) NPM can poll - SolarWinds Worldwide, LLC. Help and Support
Have you tried using the MIB browser on the device?
What I mean is that somewhere it might have been saved in the database? I mean it won't always discover it again and again. It might be saved somewhere and it polls that particular OID. Isn't that how it might work?
@jxChappel - No I don't have MIB browser. The thing is, I want the exact OID of different devices to make a list. The other option is that I have to google and browse through a lot to figure out memory OID and I have a lot of different brand devices. I got most of the Cisco devices covered with SNMPWALK.