Hi, judy,
First, check this link to determine whether your particular model of Olympus camera is supported by iMovie:
iMovie supported cameras - Apple Support
The .MOV extension designation is a container that can contain various types of codecs. The .MOV version from your camera or Android phone may contain codecs that iMovie does not support. If so, you can download the movie to your desktop (see below) and use the free download, Handbrake, to convert the video to Mp4/AAC that iMovie likes. It is easy to do and Handbrake will do it by default. You can download Handbrake at this link:
https://handbrake.fr/
Even though High Sierra and iMovie 10.1.9 (that I assume you are using) can edit the new HEVC video codec from your iPhone 8, it possibly still might be the cause of the problem. You can adjust your settings on your iPhone 8 to change the format to most compatible. Take a look at this recent link:
import videos from iPhone 8 onto my Mac
Additionally, instead of importing directly into iMovie, try importing to your desktop using the Image Capture app on your Mac. Then try dragging the imported movie into iMovie. You also can use the imported desktop movie to make the Handbrake conversion mentioned above. Another way to import into iMovie is to import into the Photos app on your Mac, and then you can access the video from within iMovie using the Photos Library item in the iMovie media sidebar. That actually is the recommended way of doing it.
-- Rich