http://www.openbsd.org/faq/faq4.html says
Install: load OpenBSD onto the system, overwriting whatever may have been there. Note that it is possible to leave some partitions untouched in this process, such as a /home, but otherwise, assume everything else is overwritten. NOTE for re-installers: The new installer will not clear your old disklabel if you chose "(C)ustom Layout", but you will need to re-specify each mount point using the 'm' option in disklabel(8). when re-install OpenBSD, if we want to keep original /home's data, are following steps the key point? 1) let the space belong to original /home partition untouched. 2) not set /home mount point, add /home to /etc/fstab after install If I have more than one partitions to leave untouched, to save more steps & time, I choose "(E)dit auto layout" rather than "(C)ustom Layout"#,delete some partitions, add some partitions carefully and leave mount point untouched. the steps are not simple and error-prone, should we improve the installer ? before the step " Use (A)uto layout, (E)dit auto layout, or create (C)ustom layout? [a] " we add some step, for example: Keep some partitions untouched? [yes] Enter Partitions' mount point? ('?' for list) [/home] ## Note that multiple mount point can be listed, separated by spaces. ...