The OG Xbox is the Grand Daddy of the modern modded console. This thing was blown wide open very early on and with it's internal hard drive and similarities to modern Pcs at the time made it a prime candidate for some excellent homebrew / emulation.
The first thing you'll want to do is get your OG Xbox Softmodded.
Then you'll want to TSOP flash it if it's a 1.0 - 1.5 (REMOVE THAT CLOCK CAPACITOR WHILE YOU'RE IN THERE)
If you're it's a 1.6 then you're going to need a modchip.
I'd recommend an OpenXenium which you can find on ebay as well as an
LPC Rebuild Board from Chimeric Systems.
Here's a video where I softmod, Tsop, and flash an EvoX bios to an OG Xbox
1.6 OpenXenium + LPC Rebuild install