You are correct in the fact that oslevel will give you the current installed version, but that is not always enough information particularily if you are asked the question by support personnel.
# oslevel <--- this will only give you the Base Level
To be more precise you should use the following command which will give you additional Technology Level, Maintenance Level and Service Pack level information.
# oslevel -s
5300-09-02-0849
This will give you
"5300" - Base Level
"09" - Technology Level
"02" - Maintenance Level
"0849" - Service Pack
On some older versions of AIX the -s option is not available in whichh cas you should use the -r option which will report as far as the Technology level
Best Answer
I believe the "lslpp" command would give you what you need, or even "smitty software".
Will you show you all software packages installed.