Normally running Oracle on AIX 5.3, you need to do some memory tuning so that you don't "double up" in buffering files. General rule is to reduce AIX amount of mem used to buffer IO and put this into Oracle's file buffers (in the SGA). This is because Oracle knows better what to cache, as it is its own data.

I don't recommend changin the following parameter by itself but it is (IMHO) the most important new parameter for an ERP/OLTP server (Introduced in AIX 5.3 ML1 and AIX 5.2 ML4).
vmo –p –o lru_file_repage=0

This means that program memory requests (e.g. Oracle file buffers) get priority over AIX's own file buffers. This is the behaviour you'd want.

Of course there are a whole heap of other parameters you'd want to check so please DO NOT just run this command without understanding what the other settings are.

And the previous post was right, just looking at "free memory" in AIX doesn't tell you much at all as AIX will try to use all memory all the time.

I hope you are using DIO or CIO access from Oracle if using JFS2 to hold your data (and DIO if using JFS). Also Asynch I/O.