What MOS should I consider reclassifying to in the Reserves that is related to my civilian police officer job?

As PO1 H Gene Lawrence stated, the MP route is the obvious choice if you're looking for overlap/redundancy with you civilian career (31B/police officer, 91B/auto mechanic, etc.).

Other options to explore that are "additive to your civilian career" (assuming you're not a 'specialized' law enforcement officer) like the "other 31-series MOSes" such as Military Working Dog Handler (31K), Corrections/Detention Specialist (31E), etc.

Finally, there are ones that are "in the same ballpark, but different sections of the bleachers" (aka "same church, different pew"), such as a 27D|Paralegal Specialist (much more involved in the 'other side' of the legal system) or a 35L|Counterintelligence Agent (does much of what would be considered "detective work" or "specialist" work in civilian law enforcement).

My opinion is this; choose an MOS that you are interested in and that you think you will enjoy. Outside of some very specific categories such as aviation, IT and chaplaincy/law/medical (in my opinion), there is never a true correlation between a military job and civilian job, especially in a Reserve Component capacity.
I am a 13F in the Reserve (formerly in the Guard) and a civilian Police Officer. I love both. I like that they are different and are separate. To me, I get a break when I go to drill and get to do something different than my day-to-day civilian job. I definitely learned some intangible and soft skills from my MOS and through deployment experiences which have helped me in my Police career. But it is the fact that I enjoy both and am passionate about both that helps me be successful.
Two of my best friends from college were Active Duty MP Officers out of ROTC. One served on a PSD team and as a Provost Marshal overseas during his deployment. The other served his entire AD time as a patrol shift supervisor for Fort Drum's Provost Marshals Office and part of their SRT. Both are now civilian Police Officers. From what they have said, it is more that they simply served in the military and were enlisted infantrymen, than the fact they were MP Officers, that helped them get hired and excel in their current civilian jobs.

I would only add to this, that some larger depts we deal with here, like LAPD, LASD and SAPD prefer any MOS other than MP (they feel MP teaches the wrong habits). Your mileage may vary where you work/live.

But yeah, an ancillary 31 series MOS, or 35M HUMINT (working sources, same as a cop with different words and different incentives), or 68W medic (with EMT basic), or 68X behavioral health specialist (for those tougher calls), can be handy as a cop.
Of course, with all things Guard/Reserve, temper your choice with distance to that new position.
For OCS and a basic branch, same, consider the distance to the unit, and consider sticking with anything that has BOLC available same year, you would be waiting longer for AG, CM and OD for dates, Signal, CYBER and MI for your TS and then dates after you have that.