This article assumes you read this article and know how to run a live webinar with OBS. If not please read this first.
Open up your Microsoft Powerpoint slide, and from the top ribbon, click on "View" > Select "Reading View" (Reading view is deprecated in newer PowerPoint versions, if you are using a newer version, in that case, you have to resize your window in OBS, so it just shows the slide area). Make sure that your PowerPoint app is NOT minimized.
Open your Open Broadcaster Software (OBS), go to "Sources", click on "+" icon and choose "Window Capture". Give it a name and from the next popup's "Window" menu, choose the "powerpoint.exe" app. Once done, you will see the powerpoint slide in the preview area.
Here is an example of how it ran for me on Youtube Live. So in case you are also wondering whether your transition animation will work this way or not, it certainly will.