If you want to show a PowerPoint as part of an OBS scene but have no additional monitor for a display capture of that fullscreen show, you may consider running that slide show as a window as shown below and then do a window capture.
1st problem: You will always have to fine tune that PowerPoint window's size to match exacty your OBS scene.
===> PPT Size & Control will force the active PowerPoint window size to precise 16:9 (1280 x 720).
2nd problem: if another window has the focus, you cannot do "next slide" without bringing it to the foreground and clicking around.
===> PPT Size & Control will intercept the ↓ and ↑ keys (with a global hook) and make sure they're always routed to the active PowerPoint window.
Both resizing and intercepting can be turned on and off - by default (program start) they're both active.
Actual PowerPoint setting to run a show in window mode rather than fullscreen: