Your stream overlay is going to be most likely an image or a video, and in some cases it could be a browser source. I will show you how to all 3 to OBS studio so you can decorate your stream the way you want it.
Stream Overlay Image
In the 'Sources' section you will see a + button, click on it to add a source. You will see all these options, click on 'Image'.


Stream Overlay Video
The idea is the same for an overlay video. Click the + to add a new source and choose 'Media Source'. Choose 'Browse' and navigate to your video file and select it.


If you are making your own stream overlay loop:
It's better to make a short video that loops than a very long video. I made this mistake when I first started. I rendered a 20 min video as a loop. Not exactly sure why but just render a video that is one loop, the shortest loop possible. It's better for performance.
Stream Overlay Browser Source
First off, you may be saying why would you do this? This is a bit of an aside but when we sell overlays most people want some kind of customization, change some text, change some color, etc... If the overlay is a video I have to go into the software and re-render with their changes and it turns into a nightmare, I hate doing it.
A browser source lets me bake in the customization, so people can change the text, change the colors as they want. It's code and not a rendered video. Anyway...
Click on the + in Sources and choose 'Browser Source' and in the browser source URL add the URL. In this example I'm using our Halloween vertical overlays on StreamElements so to get the URL of a StreamElements overlay click on the little link icon here:

And set the height and the width, I always set to 1920x1080 and then just adjust anything I need in the canvas. Then click 'OK'

