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.

yes.

You are assuming that 1) the disklabel was previously done by using "auto
layout" and wasn't edited, and 2) the disklabel command in the new install
kernel uses the same partition sizes and layout as the command used for the
previous installation. This is not a safe assumption.

Having this in the installer would be "blessing" something dangerous.
IMHO: if you want to keep data do an *upgrade* not an *install*.
For anything else use dump/restore.

This is exactly what happened to me when doing a fresh install on an old
disklabel.

fsize and bsize.
But it all worked out in the end.

If you want to preserve anything, back it up.
Write down every number in disklabel, or forget getting it back.
I had room for adding another partition at the end so I put a copy of
everything there also.

Chris Bennett