- Kernel Mode Setting for Radeon is in -current, and will be included 5.5-release.
- All that is needed is Radeon firmware and a -current system. No special action is needed for supported chipsets.
If you are a -current user, see this entry in the Following -current FAQ which described actions for -current users to take when KMS for Radeon initially became available. There, you will find a link to the firmware. http://www.openbsd.org/faq/current.html#20130812
If you are unfamiliar with -current, all you need is the firmware and the latest snapshot, but before using it, please review FAQ 5.1, which explains the flavors of the OS. http://www.openbsd.org/faq/faq5.html#Flavors
Note that at this time, -current is in beta test for 5.5-release. When the Beta ends, 5.5 will be tagged and frozen; -current will then move beyond it.
Edited to add:
Upgrading from 5.4 to -current requires a significant effort. If this is a test system, installing will be easier. See http://daemonforums.org/showthread.php?t=8048