Interest in supporting the MIPS architecture in BSD is about as old as the architecture itself, and Risc/OS (the Unix variant running on MIPS' own workstations, before MIPS got bought by SGI and stopped manufacturing anything but processors) was based upon BSD code; so was Ultrix, Digital's first flavour of Unix, which ran on VAX but also on its MIPS-based DECstations.
While there is a lot to tell on OpenBSD on SGI hardware alone, I think it is better to see a larger picture. This story is quite long, and has been split into six installments.
You can also read it all in one (big) page.