Yep, you spot one up, just like regular One Pocket.
However, if you want to set the rules so the game cannot extend too much, you can put a foul limit -- such as, if you already need two balls and you scratch or foul again so that you would then need three balls, that would be loss of game. If we had official One Ball One Pocket rules, I'd push for this to be in them, btw. So the most balls you can need would be two balls to keep the game moving forward. A couple of hangers in your opponent's pocket too deep to take out could easily do that to you, lol.
You also of course need to agree on where the cue ball and object are placed to start the game, and there are different reasonable options for that.
But one other rule you need is that you have to play a safe for your opening shot, you cannot just pocket the ball -- you don't have a game if the opening player is allowed to pocket the ball on their first shot.
Other than these simple rules, it's all just ordinary One Pocket.