Mine is just "consistent system logic".
My card game has "templates" for cards to provide an "internal logic". X race has Y default stats... which change by A, B, or C when specific Jobs are applied to that race... which get 1 of 5 effects based on rarity, or get D, E, and F stats added to other stats as well.
The "internal logic" tends to keep me from engaging in "power creep" among other things. Like "implementing any and every fool idea I have".
I engage in this in RPG design as well. Each skill is a "template" with specific values that go up or down depending on what it does and what it's "tier" is meant to be. Monsters use a similar template... etcetera.
Someone good at spotting this stuff while playing would probably notice the templates and be able to work out the "rules" I'm using, but I have to have these "rules" in order to make things "work". Otherwise, I'd get wildly inconsistent results and trying to figure out how to balance that is a pain... so I just work with the "templates" in those situations.
As for other OCD stuff...
I do a lot of "cut the fat". It's something I simply have to do. If I can do something one of 3 ways, i will pick the quickest and most efficient method of implementation so long as it works and doesn't have any negative drawbacks I don't want. I don't like to "overcomplicate" things. I'll cut systems that just aren't efficient or eat up a lot of time to implement. I'll slash coding and variables that spend too much time calling things and referencing things. I try to make the "coding lines" as short as possible to reduce the workload and minimize possibilities of bugs.
I just like things to be as efficient as possible. Nothing unnecessary included. All waste removed. All pointlessness excised. If it has no purpose or doesn't serve the purpose it was meant to, it's gone. I have no need for it.
The problem is this "cutting the fat" often translates to wanting to do it everywhere. Including in other people's projects... and even in running things at workplaces. Probably doesn't help that this "cutting the fat" thing is basically why people hire me to do the jobs I do... and why my workplace relies on me so heavily.
But, it is difficult to see the inefficiencies and not want to go, "move aside, let me fix this! LET ME REMOVE THIS NONSENSE! I DON'T CARE IF YOU ARE SENTIMENTAL! IT IS A WASSSSSTTTTTTTEEEEEEEEEEEEE". Very hard to not indulge in this impulse.