oracle 8.1.5 has been 'out of service' for some time, but as it is 32bit, it should still work on AIX5.2 as the AIX 32bit runtime environment hasn't changed since AIX4.3.3.
oracle relink all still is a good idea anyway.
I run 8.1.7 databases on AIX5.2 for an SAP system that has been stagnant at SAP4.6/ORA817-32 for some time. Will probably go to 9i-64 in the near future...
Make sure you keep the AIX kernel at 32bit, because you need to install kernel extensions for your oracle software that won't fit in a 64bit AIX kernel.
if you set vmtune attribute of sys0 to pre-5.2 then you can keep on using your /etc/rc.vmtune script or the like, but you will have to modify it
The vmtune sample program has been replaced by vmo and ioo commands. If you have an output of vmtune on 4.3.3 and run vmo -L and ioo -L on 5.2 you'll get the idea which tunable needs to be set. vmo and ioo are a bit picky on which tunable you set first, because there are some dependencies.
HTH,
p5wizard