Personally I don't know enough about the current state of OHS or AIX to comment.
The question of installing OHS standalone (prior to implementing APEX) may be more suited to the Oracle Application Server - General forum.
I have checked an oracle HTTP server FAQ before launching the installation and noticed tat mod_plsql (which is needed by apex ) wasn't apparently supported by HTTP server based on Oracle Application Server 10gr3 CD companion am using.
The only version that supports mod_plsql is on the Oracle Database 10g Companion CD Release 2 (10.2.0.1.0) that has OHS based on version 1.0 of Apache.
According to the Doc ID 1071023.1 I should request the url for the cd through an SR under problem: "Software & OS Media Requests". I'll get back here if things work (didn't know it was the wrong forum).
however ,one can install a separate HTTP Server 11g with mod_plsql (Oracle Notes: Install Oracle Http Server 11g (220.127.116.11.0)) and configure it to access Apex which is installed in the 10.2.0.5 database as per the steps from http://docs.oracle.com/cd/E37097_01/doc/install.42/e35123/otn_install.htm#CHDHCBGI
1.5 Is Apache v2.0 version of OHS supported with this release?
Yes. However, OHS based on Apache 2.0 is only supported in a standalone deployment version. It
has the same functionality as OHS based on Apache 1.3 except for the following:
• IPv6 is supported in OHS based on Apache 2.0 but not in OHS based on Apache 1.3
• mod_oradav is not supported in OHS based on Apache 2.0
• mod_dms is not supported in OHS based on Apache 2.0
• mod_plsql is not supported in OHS based on Apache 2.