How do you compress a video on OpenCut?

You compress a video in OpenCut when you export it: click Export, pick a lower Quality setting, and if you want it smaller still, pick WebM instead of MP4. The export menu in OpenCut's own code has four quality options, Low for the smallest file, Medium for balanced, High (the default and the one it recommends) and Very high for the largest file, plus two formats, MP4 (H.264) for better compatibility and WebM (VP9) for a smaller file size. There's also an Include audio in export box, and if you don't need the sound, unticking it drops the audio from the file.

What I'd do is export one copy at High first and see how big it is, then try Medium and check the text is still sharp on your phone before you post it. I'd stay on MP4 unless I knew exactly where the file was going and that it takes WebM, because MP4 is the one marked for better compatibility. Shorter is the other trick: cut the dead air with S to split and Delete to drop the piece, and the file shrinks with it. My full OpenCut walkthrough covers the cut and the export start to finish.