The information here has been refined and documented in a dedicated page for Multi-Booting Windows 10 and OpenBSD. That page has all up-to-date details, and this post is no longer the best available source of information on this topic.
I recently purchased a 13.3" 8th Gen Lenovo ThinkPad X1 Carbon. Frankly, the X1 series has been my dream machine for years. I like small laptops, and this one is light, powerful and is similar to what's used by many of the OpenBSD core developers, so I knew it would probably be well-supported. My previous laptop -- the Acer I upgraded to an i5 for VMM years ago -- was set up for dual-boot, but somewhere along the way, the Windows boot manager stopped booting OpenBSD so I'd been using a modified OpenBSD install image on an SD card to load the OpenBSD kernel from the internal SSD, I set the BIOS to prioritize the SD card for booting, and I just remove the SD Card from my Acer if I need to boot Windows. That laptop is growing long in the tooth, but it's served me well for the past 5 years.
I decided to try properly dual-booting Windows and OpenBSD again with my new ThinkPad. And that's where things got ugly.
The steps to get Windows and OpenBSD working together, as outlined in the OpenBSD Multibooting FAQ seem to not work at all on recent Windows 10 versions, and especially on modern PCs with EFI and GPT disks. I tried several times without any luck, and I also rendered my system unbootable a number of times in my quest. Fortunately, I had made recovery media so I could blow away my X1 Carbon to factory defaults when things went sideways. That's a 45 minute process each time.
Start with a Windows install, and have good backups, including, if possible, external recovery media from the manufacturer. Several times, the drive partition table was so screwed up that the recovery partition was missing as well, leaving me with the recovery USB stick as my only way forward. To resize the Windows partitions, you will have to completely disable BitLocker full-disk encryption if it's enabled. I am a fan of FDE, but we have to turn it off for this to work. You can re-enable it when you have your whole system back up and running, but there are come caveats at the end.
Create a Live USB of GParted.
On another USB stick, write the contents of the OpenBSD installXX.img. This link references install68.img from OpenBSD 6.8, which may be out of date or a broken link when you read this.
Boot into the gparted live distro. On modern EFI/UEF systems, you will probably need to adjust secure boot and/or legacy boot options in your system's BIOS to continue.
Shrink the main Windows partition by some amount to make room for OpenBSD. I gave myself 120GB. That's how much room I have dedicated to OpenBSD on my Acer, and it seems to be a good size. I also usually create another FAT32 or Exfat partition that I can store files on to be accessed from both OpenBSD and Windows, but that's beyond the scope of this write-up.
Create a new partition for OpenBSD in the empty space. GParted doesn't know about OpenBSD partition types, so you'll have to then close GParted and launch a terminal window from the live environment. You'll have to launch gdisk via sudo and address your drive's device. For example:
sudo gdisk /dev/nvme0n1
Use the "p" option to print a list of partition entries. Find the one you created and use the "t" option to change the partition type to "A600" which is what OpenBSD expects to use for its disklabel entries. "w" will write the GPT and exit. You may also want to use the "b" option before you exit to make a backup of your partition tables just in case you mess something up. You'll have to store it to a USB drive, but you can probably store it on the same USB stick you booted GParted from.
At this point, I decided to reboot and make sure Windows still works. Thankfully, it did. Reboot into the OpenBSD installer using the USB stick you created. I won't walk through the whole installer process, but pay very close attention to the disk partitioning options. When prompted for the disks to install OpenBSD to, you should see an "OpenBSD Area" option and that should be the default disk partition to install to. If that option doesn't exist and you choose "gpt" or "whole disk" you will destroy the GPT record on your drive and destroy the Windows installation. Your system will only boot into OpenBSD if you proceed. You can probably use gdisk and your backup of the GPT to recover the partition table if you didn't actually install OpenBSD, or you may have to reinstall Windows and start all over again, and that isn't fun. Trust me. I've done that four times this week. Don't let the system do an auto-layout. Choose "custom." Unless you know what you're doing, just make one big disklabel partition for OpenBSD's root drive.
Once OpenBSD is installed, exit to the shell. You will need to copy the EFI boot executable to some other media so you can access it from Windows. I inserted a USB stick that was formatted for FAT32. It showed up as sd1 and the first non-BSD partition typically shows up as "i"
mkdir /usb
mount /dev/sd1i /usb
cp /mnt/usr/mdec/BOOTX64.EFI /usb/bootx64_openbsd.efi
umount /usb
Go ahead and reboot. It should boot into Windows. Fingers crossed!
Now, go download rEFInd and unzip the archive. I followed the Windows manual install instructions to get rEFInd working. I rebooted again, simply because I'd become so accustomed to bricking my shiny new laptop. To my surprise, rEFInd presented me with a boot menu. Windows showed up, and an additional menu item called "Fallback boot" also appeared. This menu option booted me into OpenBSD. That could be pretty much the end of it, but I wanted an actual OpenBSD menu option.
To accomplish this, I borrowed some mojo from this somewhat dated blog entry on FunctionallyParanoid
You have to access the EFI system partition from a privileged command shell (hearkening back to the instructions to manually install rEFInd from Windows), so I copied the refind.conf file off to my Documents folder, edited it with notepad, then saved it and copied it back over to the EFI system partition.
I added this clause near the end of the refind.conf file:
menuentry “OpenBSD”
{
icon \EFI\refind\icons\os_openbsd.png
loader \EFI\boot\bootx64_openbsd.efi
}
Make sure to download the OpenBSD icon and place it in the \EFI\refind\icons folder, too. Once I did that and rebooted, rEFInd still had the "fallback" menu item, but OpenBSD showed up with its own logo alongside the Windows logo. Both operating systems boot, and my mission was finally accomplished.