You can insert a YouTube video in a PowerPoint Online presentation, and then play it in Slide Show.
Select the slide where you want to place the video.
On the Insert tab of the ribbon, in the Media group, select Online Video.
In the Insert Video dialog box, type search keywords to specify the video you're looking for, and then press Enter.
The results of your search will be shown. Point the mouse at a thumbnail image to see the title of that video.
Select a video thumbnail image and then click Insert.
The video is inserted on your slide. You can move the video by dragging with the mouse, and you can resize the video frame by selecting a corner and dragging.
To play the video, you must run Slide Show. On the ribbon, switch to the View tab, and in the Start Slide Show group, select From Current Slide.
Slide Show will open, and then you can click the play button to begin.