Air traffic approach controller reported an aircraft transitioning the airspace descended toward another aircraft on final approach causing the aircraft to descend below the MVA to avoid the descending aircraft.

Narrative

I was working North radar with everything combined. Traffic was not complex. I was vectoring Aircraft X to final for visual approach RWY 9R at OSU. I had been watching an unidentified VFR target depart from DLZ and go southwest towards the OSU final floating between 3500-3600 feet. I figured at that altitude it would be ok to bring Aircraft X in on the approach at 3000 feet below the unidentified aircraft and issue traffic. I issued a heading to join the 9R localizer and continued to issue the traffic to Aircraft X. I noticed the VFR aircraft started to descend out of 3500 feet and told Aircraft X to expedite his turn. The Conflict Alert (CA) went off on the scope; I issued a traffic alert but did not issue an immediate turn due to the fact the fact I already expedited a turn to avoid getting Aircraft X away from the target. He stated he was responding to an RA to descend which put him below our 3000 feet MVA so I issued a low altitude alert to the aircraft as well. Aircraft X after that low altitude alert statement; stated he had the field in sight; so I cleared him for the visual approach clearance since the traffic was no factor any longer.I believe there needs to be a NOTAM statement around OSU airport restricting aircraft from navigating or practicing around the final approach courses at a busy airport like OSU. ATC needs to be in contact with aircraft within a 10 NM radius of OSU to help avoid conflicts like this one. This situation happens a lot where VFR aircraft with not talk to ATC and trek through the final approach courses at altitudes IFR aircraft require to shoot a stable approach to the airport. This is unsafe and will continue to have RAs along the final approach course to this airport.