your best bet (and probablly easiest) would be redhat 6.2. i believe that's what oracle for linux is "officially" supported on, so installation might go smoothest. it should run on other distro's fine but will probablly just be easier on redhat.
you can download it, you can probablly find mirrors somewhere off [url]http://www.redhat.com[/url] , but i find it's easiest just to pick up a cd and boot from there. my floppy disk hasn't worked in about 4 years so don't really have a choice though. if you download and make a boot disk it shouldn't be too bad.
oracle for linux is a free download for development use anyway.
although i haven't used it personally, i've heard to stay away from redhat 7.0 as it's incredibly buggy. 6.2 has been fine for me.