For my livestream, I'd love to automate PPTx slide advances, but Microsoft changes things in recent versions, and really a challenge.. so I've stuck with Manual slide advances
So, you can manually advance PPTs (or interact with any application window as normal.
What I suspect you didn't say is that you have a single monitor, and are doing a Window/Display capture that prevents having PPTx visible?
So, the issue is that OBS is designed as an inter-active compositing tool. There are much simpler screen recorders, is that is all you are doing
Personally, I simply got a 2nd monitor as it makes things SO much easier.
The other thing I did, planning on 1080p stream/recording, was to get a 1440p monitor (a 4K would work just as well, just more expensive) and I run PPTx slides in a 1080p sized window, vs full screen. Then I set up OBS to capture the 1080p Window, meaning I can have content visible on top of the PPTx slide, but OBS' capture of the slides works as desired. I leave a little portion of the PPTx Slide Window visible, and chose an older OBS Capture method so the cursor isn't visible... now I can mouse over the visible area, use the scroll wheel on the mouse to change slide, and continue my primary interaction with OBS or other items (in my case, PTZ camera, monitoring livestream browser page, etc)
As a side note, there is the reverse approach of enabling web-sockets in OBS, and using Macros in a PPTx slide to send commands to OBS
The other thing folks will do if sticking to single monitor (especially a laptop in situations where a 2nd monitor isn't practical) is to capture the monitor (slides, in this case) and use something like a Stream Deck to provide push-button control of OBS