Holy smokeballs, thank you so freaking much!!! This thread caught my eye as I've been wanting to customize my Zoom and MS Teams "stream" (ie. webcam + extra background/overlay goodies). I hate to say it, but I skipped past the OP's project and tried this suggestion of NDI first.
Amazing. It took a good 30 minutes to figure it all out, but I now have the output of OBS (in my case, just the preview itself without needing to stream/record) as a video/webcam input source for Zoom, Microsoft Teams, and within Firefox. Note: doesn't work with Discord or QuickTime's File > New Movie Recording.
And all this with me being on macOS. Not Windows, but macOS. Incredible.
Steps (should work for macOS and Windows, not sure about Linux):
1. Install OBS. Run it, and set up a basic scene for testing (eg. webcam and a text label).
2. Download/install NDI Tools for your OS from https://ndi.tv/tools/#download-tools (note: system restart required). You only need the "NDI Virtual Input" app; on macOS each app had its own .pkg file bundled in the single .dmg archive; on Windows I assume it's an install wizard with checkboxes for each component. Again, only need "NDI Virtual Input" app/component.
3. Run the NDI Virtual Input application installed in step 2. It should live in your systray (without doing anything useful yet).
4. Download/install the obs-ndi plugin for your OS from https://github.com/Palakis/obs-ndi/releases - right now for Windows or macOS it's version 4.9.0 (expand the "Assets" link). There's a 4.9.1 update specifically for Ubuntu/Debian, but I'm not sure how those OS's are supported when there is no NDI Tools for Linux in step 2.
5. Run OBS. If you're lazy and didn't read the GitHub release notes in step 4, starting OBS should popup with a direct link to the NDI runtime you also need to download/install; then restart OBS.
6. In OBS, go to Tools > NDI Output Settings. If you want to "clone" the OBS output to the NDI virtual device only when you start streaming/recording in OBS (ie. to disk or to a streaming platform like Twitch), check "Main Output". Otherwise check "Preview Output", in which case your OBS preview will be output to the virtual video device at all times without having to start OBS streaming/recording.
7. In your operating system's systray (ie. top-right on macOS, bottom-right on Windows), you should have an NDI icon living there as started in step 3. With OBS running and configured according to step 6, you should be able to click the systray icon and select that output source as the input source for the NDI virtual device.
8. Open Zoom, Teams, or hopefully other apps which will work. Wherever you configure which input source to use for video/camera within that app's settings, there should be an "NDI Video" source. Select that… and BAM – your OBS canvas is now your input source!!
That took so long to type out, I hope someone manages to make use of it. :)
Yay!!! There were a couple of steps that took me time to debug how to make it work, so I'm ecstatic that at least one person found my steps useful!!! I don't have recent experience with Google Hangouts, so I'm curious to know whether that was a native app (does Hangouts have a macOS app?), or which browser you used (Firefox, Chrome, or…)?
Sadly I can't get this to work on macOS. OBS works, NDI seems to be installed and configured. It even shows up in Chrome, but I can't select it as an option.
Just for you I installed Chrome to check. It worked for me. I wrote a few things to check below; I suspect your problem is item 'c'.
a) Make sure Chrome has access to cameras (System Preferences > Security & Privacy > Camera > Google Chrome checked).
b) In step 6 from my comment, make sure to try "Preview Output" in OBS's Tools > NDI Output Settings, as "Main Output" requires you to start OBS streaming/recording (which you probably don't want to do).
c) After setting up that OBS setting, clicking the NDI systray icon should show a dropdown list with a single item. You actually need to click that item in the list (it will checkmark it). If you don't do this, you'll get a black screen when trying to pull from NDI.
I couldn't find an NDI product for macOS from NewTek.