Multibooting OpenBSD

I've used this method from 5.6 up through -current (while 6.2 is the release).

According to the OpenBSD FAQ, multibooting is non-trivial. Despite their somewhat dire warnings, it's not that difficult if you know how to do it. The usual caveats apply, it's always better to have backups, there is always the risk of losing what is already on the disk.

This is assuming the newcomer has a bit of knowledge, a machine with Linux installed, and a free partition. It does not have to be a primary partition, a logical partition inside a Linux extended partition works just as well. I've used this method with OpenBSD-5.6 through snapshots following 6.2.

Once one has said free partition, begin the OpenBSD installation. When it comes to what part of the disk to use, you can choose e, as in edit. In our example, we have a 4 partition MBR layout. We will use the last partition, partition 4 in Linux fdisk. It will show as 3 in OpenBSD's, with partitions labeled 0-3. Once we've chosen e, we will see fdisk: 1>. We then type 
edit 3

3 is the Linux /dev/sda4 partition.

It will print the disk id (in this case, the filesystem type, which would be 83 if it had been a Linux partition), and some other information such as cylinders, heads and sectors. Type in a6, as we would in Linux fdisk. It will ask if you wish to edit in CHS mode (Cylinder, Head, Sector) with a default of n for no. Hit enter to take the default. It will then show the offset (start) and size, which will be what you see if you had just typed p for print. Accept the defaults given. Now hit p to be sure it's what you want. It should now show partition 3 as id A6, OpenBSD.

If you're confident of your changes, you can type write and then quit. (Or just quit, either one will save the changes). Typing exit will exit fdisk without saving changes. Many other programs use opposite terminology, exit saves changes and quit discards, so be careful. You can also type abort, which will, as expected, just exit the program without saving changes, though if this is done during installation (as opposed to using a shell and just typing fdisk -e sd0, the OpenBSD terminology for the disk), it won't exit completely, it will bring you back to the dialog of whether you wish to use the whole disk, or edit.

OpenBSD can also be installed on an extended partition. In that case, it may be easier to use Linux's fdisk prior to installation. In this case, let's say we have 3 primary and one extended partition, with, in Linux terminology, drives 5 and 6 as logical partitions. We're going to use partition 6 for OpenBSD. So, while in Linux use the following sequence

fdisk /dev/sda
t  
6 
a6  
w

Toggle the type of partition
Choosing partition 6 in response to fdisk's question
a6 is the hex code number for an OpenBSD partition. If using gdisk, rather than fdisk, the code is A600
write your changes.

You now have an OpenBSD partition. At this point, you can reboot with your OpenBSD install media, whether it be USB, CD, or anything else. After being asked the usual install questions, after it chooses the disk and asks if you'd prefer to use DUIDs rather than device names in fstab, you should then see a listing of your partitions, including an OpenBSD partition. The choices should be 
Use (W)hole disk, use the (O)penBSD area, or (E)dit the MBR [OpenBSD]

or something similar. Choose o, (case insensitive). That's it. The next screen allows you to edit the partitions within the OpenBSD slice or use installer auto defaults. Regardless, any changes you make to the disk will only affect that partition. Assuming you boot from grub2, used by most Linux distros these days, after installation, boot back into Linux and create a /boot/grub/custom.cfg (or /boot/grub2/custom.cfg in RedHat and RedHat based systems, such as CentOS and Fedora) that reads, if you installed to the fourth partition. 
menuentry "OpenBSD" {
set root='(hd0,4)'
chainloader (hd0,4)+1
}

If you used partition 6 as we did with Linux's fdisk, you would change the hd0,4 entries above to hd0,6. If you reboot and select that menu entry, you will then boot into your OpenBSD installation.

This doesn't work on UEFI installs. I've only done this once with UEFI, and it's a bit of a hack, but it works. This is a laptop which uses grub2 from VoidLinux. In its grub.cfg it has a section that reads 
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply
type the
# menu entries you want to add after this comment.  Be careful not to
change
# the 'exec tail' line above.

OpenBSD creates bootx64.efi which it places in /boot/efi/EFI/BOOT. After a bit of web searching, I came up with 
menuentry "OpenBSD" {
        insmod part_gpt
        insmod search_fs_uuid
        insmod chain
        set root=(hd0,gpt1)
        chainloader /efi/BOOT/bootx64.efi
}

I'm not quite sure why that path works, the directory is actually /boot/efi/EFI/freebsd, but at any rate it does work. At some point, I'll figure it out, I suspect. If I do, I'll update the page, but at any rate, multibooting is not as difficult as the OpenBSD FAQ implies.

In case it's not clear, the OpenBSD chainloader points to the first partition, which holds the efi file, it isn't worrying about which partition holds the OS.