First, you plan what to do within the application and record your application with OBS.
Then you use some video postprocessing software ("Video editor") to edit and optionally re-arrange your video recording, as well as a adding your narrative to the existing video. Some video editors have an integrated feature to record a narrative. If your video editor doesn't have this, record your narrative with some other audio recording software and integrate the recorded audio track into your existing video with the video editor.
The idea is after you roughly cut and arrange your video without sound, you playback the silent draft video in your media player or in the preview of your video editor and speak+record the narrative. Then cut and integrate the narrative into the video.