Hi Anne!
Yes, there are apparently a couple of ways to do this. I can talk to my colleague Curtis about his method, which involves setting up OBS to record once you "tab over" to a particular window. The method I've been using (because I have headphones on when I'm recording, and I mute the "desktop audio" side of things, is to set up keyboard shortcuts, so when I hit "ctrl-R", it starts recording, and when I hit "ctrl-alt-R" it stops recording. The "ctrl-R" makes a big "DING" sound on my desktop when I'm clicked on powerpoint, but because I have the desktop audio muted it doesn't matter.
I'm happy to either talk to Curtis about his method, or talk you through setting up keyboard shortcuts, it is very simple.
Amanda