Sure. You can check out websites like Archive of Our Own (AO3). Just search for 'doomguy x oc' there and you'll find a variety of fanfictions. Some might focus on their adventures together in the Doom universe, while others could explore a more romantic side of their relationship.
Well, I'm not sure about specific titles off the top of my head, but if you go to fanfiction.net and look under the Doom section, you may come across some interesting doomguy x oc fanfictions. Look for ones with high ratings and lots of reviews as they tend to be more engaging. They could involve the oc character helping Doomguy in his battles against demons or having a unique backstory that ties in with the Doom lore.