The countdown scripts I could find for OBS didn't work exactly how I wanted, so I wrote these simple shell scripts to do it. There is intended for a Mac, and the scripts have hardcoded stuff based on my file locations, so if you choose to use it, you'll need to change it appropriately.
To use the timer, setup a text item in OBS, set to read from a file, and point it at the countdown.txt file generated by these scripts.
The OBS-Countdown.sh is a script that's intended for integration with BitBar. To use it, crease a Symlink to the BitBar plugins folder:
ln -s ~/obs/countdown/OBS-Countdown.sh ~/bitbar/OBS-Countdown.sh
This will give a ⏰ icon in the menu bar, with Reset and Start commands.
By default, Teams won't recognize the OBS virtual camera. To fix it, run the fixteams.sh. This will need to be done each time teams is updated.