As an ideal video editing tool, Windows Movie Maker is widely used by many video editors. The point here, you will find it is a painful experience to import OBS files to Windows Movie Maker for editing. There're some frequently asked questions quoted as below.
Q1: Can't open OBS videos in Windows Movie Maker
So I just downloaded OBS and after a couple minutes of messing with the settings I started to record. After making a test recording I opened the file to watch the video; by default it opened with "Windows Movie Maker" and as I expected it didn't work. It gave me a blank screen…
Q2: Windows Movie Maker won't work with OBS
When I load my OBS videos into Windows Movie Maker they come up as a black screen with no audio; so I was wondering if there was a way to edit my videos for free or if I'm just going to have to stream. And a note my videos come up just fine in windows media player.
Actually, it's not so hard to do the trick. Why we always have trouble playing, editing OBS files in Windows Movie Maker that's because Windows Movie Maker is not able to support OBS recordings perfectly. You just need to change the OBS .mp4/.flv clips to something Movie Maker could natively handle, and WMV format is usually ideal. Brorsoft Video Converter is just what you need to achieve your goal.
It is an optimal video converter for OBS recordings. It supports creating high quality WMV video for your Windows Movie Maker, no matter what's the format of your source video files. It can convert almost any video including FLV, AVI, MP4, VOB, MKV, MTS, MXF to WMV so you can import your video to Windows Movie Maker for editing. Plus, it comes with all regular video editing tools like Trim, Crop, Effect, Subtitle, and Watermark. For basic video editing tasks, you can even replace your Windows Movie Maker with it. Download the software and learn how to convert and import OBS to Windows Movie Maker.
Guide: Convert OBS recordings to Windows Movie Maker
1. Download, install and launch OBS to Windows Movie Maker Converter. Then click “Add Video” button to browse and choose the video files you want to convert. You also can drag the files to this program directly. Batch conversion is supported, so you can load more than one file to save time.

2. Click “Format” menu firstly and then follow “HD Video” to set WMV HD(*.wmv) format. If you want to make changes to the video settings like resolution, bit rate and other settings, click the Settings button beside Format bar.

Tip: Click "Settings" button if you'd like to customize advanced audio and video parameters like Video Codec, aspect ratio, bit rate, frame rate, Audio codec, sample rate, audio channels.
3. When everything is ready, click the Convert button to start transcoding OBS videos to WMV for Windows Movie Maker.
After conversion, you can click the Open button to open the output folder and find the converted files. Now it would be easy for you to import and edit OBS files in Windows Movie Maker.
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Source: Guide: Convert OBS recordings to Windows Movie Maker