I don't know what you've done. From what you've posted:
- There is no flagged partition in your root MBR for your BIOS to boot.
- You appear to have a valid MBR signature in your extended MBR.
- Your OpenBSD partition begins at the same track as your extended MBR, and if you install it there, you will destroy the extended MBR. Change the starting position of the OpenBSD partition to 7138/1/1 to move it 63 sectors ahead.
- None of your extended partitions are flagged bootable, either. But, as discussed above, our testing (in a virtual machine) indicate that flagging both the "root" extended partition and the OpenBSD partition will fail to have OpenBSD boot directly from BIOS anyway.