Since I've moved to Android Marshmallow, whenever I install Opera, it becomes an option in the select box that lets you pick a video player when selecting a video file to play. However, the actual video players installed in my system don't show up in this box, just Opera. When I uninstall Opera, the problem corrects itself and the video players show again.
This has only happened since I moved to Marshmallow. It prevents me from using Opera as I use my Android phone for video extensively.
Samsung S5 - SM-G900V running on T-mobile
Android 6.0.1 "Marshmallow"
Carrier - T-mobile
Opera Browser & Opera Browser Beta