Your Windows 7 machine could be using either MBR or GPT. From this Dell Help page:
http://www.dell.com/support/article/...ell-pc?lang=en
"2. How to repair your EFI Bootloader
Windows 7, 8 and 8.1 could install on a Legacy BIOS using MBR. However Windows 8, 8.1 and 10 were designed to install on the UEFI BIOS using the EFI bootloader and GPT."
OpenBSD use to provide
sysutils/grub but it does not presently build.
If you have an MBR, the OpenBSD FAQ used to have an entry on adding an OpenBSD entry to the Windows Boot loader (likely archived somewhere). Otherwise, you will be moving/resizing partitions and using a 3rd party bootloader.