Use Virtual Box to Boot an Existing Disk Partition and have Changes Saved to that Disk Partition

The answer that you need to follow wrt to VirtualBox being able to use physical disk partition is this one. I suggest you read manual for VirtualBox, chapter 9.7.1.2. Access to Individual Physical Hard Disk Partitions before following that answer.

If you correctly understand that chapter and execute the commands in accordance with your case, you should be able to access and modify whole harddisk or a specific partiton (-relative flag) with ease. I can tell you whatever you would do in the assigned physical partition from within the VirtualBox would make permanent changes. They are not temporary or in memory! I had installed my Arch Linux in a real partition by assigning the target physical partition to Virtualbox using relative flag so as to prevent myself from accidentally deleting or modifying partition table or other partitions when installing Arch Linux.

That said, I don't suggest you do such a thing at all. If your intention is to learn an OS, don't bother accessing physical partition through VirtualBox. Just take backup of your Open BSD files and save them in a virtual hard disk that you can use easily in VirtualBox. When you feel like committing fully to only using Open BSD on a physical partition then move the files from virtual hard drive back to the partition of your choice (you would have to reconfigure the bootloader though -- shouldn't be a problem).

This approach prevents you making silly mistakes with Virtualbox commands that can corrupt your partitions.