Recently i had to set-up Oracle Apex on a Standby Database Server. Below are the steps that i performed.
1] Install HTTP server from the Companion CD of 10gR2
The Installation should be done in a new oracle home.
Select Apache Standalone
Few more clicks and got it installed.
2] Copy the apex directory inside Oracle HTTP home in the Primary to the Oracle HTTP home in Standby
3] Copy the marvel.conf or dads.conf present under OHS/Apache/modplsql/conf/ in Primary to the Standby
4] To access the Apex the database should be open. It will not work in read only mode.
So create a guaranteed restore point and open the database and test and flashback to the restore point and apply logs
5] Start the http service
[$]mv dads.conf dads.conf_old
[$]mv marvel.conf dads.conf and it worked
7] Flashback the database to the restore point created in step 4 and apply the logs