The OBS video preview always runs hardware accelerated, to provide the best performance. In the Windows Settings app, go to System -> Display -> Advanced display settings -> choose your >60Hz monitor from the dropdown -> click "Display adapter properties for display" -> "Monitor" tab -> and change the "Screen refresh rate" to 60Hz. Hit Apply, OK, and you're good to go. Personally I switch this back out of 60Hz when I want to play games without recording/streaming, but that's up to you.