I tried an mp4 file, a webm file and an avi file and they all played fine in Opera GX in my Google drive by double-clicking them or by right-clicking and choosing "preview". The player that launches looks like a Youtube player.
Are the files that you just uploaded recently? It seems after uploading, it takes a little while for them to be available for playback.
Try disabling all your extensions to test if that helps. Try turning off Opera's adblocker if you're using it to test if it helps.