As Aaron indicates, OBS Studio is the wrong tool for the task you are trying to accomplish. And a simple, free video editor does sound much more appropriate. There a numerous options, Aaron's link being one of them.
On the other hand, if ALL you want to do is play video (setup as a picture-in-picture sort of view) with a fullscreen slide show behind that small video image, and manually advance slide show to match video recording.... sure OBS Studio could do that... but beware the not-immaterial learning curve of OBS Studio [then again, the video editors will each have their own learning curve as well, but probably easier than OBS Studio for this use case] . For non-gamer users, I found Paul Richards' OBS Studio [free] guides at
https://streamgeeks.us/ to be a good starting point
-
The Unofficial Guide to Open Broadcaster Software
-
The OBS Superuser Guidebook
Pointers for you if using OBS - Presuming pre-recorded video lecture includes all audio you want, then you will have 2 Sources in you OBS Studio Scene
- Window (or Display) Capture of PowerPoint slide show.
easiest if this is on a 2nd monitor, and doing full screen slide show.... though you don't have to do that. I use PowerPoint with OBS Studio in a Windowed slideshow. To avoid scaling impacts to video resolution, assuming you plan a 1080p30 video, having PPTx full-screen on a 1080p monitor would probably be best/easiest
- Source # 2 will be you lecture video.
- There are options, but the simply one is to let OBS Studio 'play' your video (vs using a video viewing application at Operating System level)
- Read the user guide/manual on how to adjust source display size (ie.. how to re-size video (Source) window into small picture in picture window in OBS Studio's Preview window, presumably in corner of frame
- This video does NOT need to be positioned on same monitor as PowerPoint. The combining (compositing) of the different sources happens within OBS Studio.
Other recommendations to keep things simple
- I'd avoid OBS' Studio Mode as that is 2X the rendering workload, and you most likely don't need the complication
- Assuming not adding any new audio, instead using only that audio which is already in the video, then Disable/Mute Desktop Audio as an OBS Studio Source. Your only 'audio' source should be the video.
Be sure to read the Guide's on audio, and understand the difference in Advanced Audio Properties (or any given Source) of Monitor vs Output