Hi Chris,
I'm not aware of anything that could do this directly, but if you choose Window capture source and it will bring up apps on your computer. i.e. Powerpoint, Keynote or whatever app is open. You can then position the app side by side with the OBS window when working on the app. While working on the app itself your changes will appear in the OBS window. Hope this helps.
Open app of your choice
Open OBS source
Choose Window Capture
Look for your app in the list. Choose it and you should be good to go.