I understand you declined this issue in a previous post, so I'm reopening it here. This is important to your users and saying you are focusing on the "most common" formats is not a reasonable response for most of us. Please reconsider or expand upon your reasoning for not supporting this.
Mkv is indeed one of the common format and it's not reasonable that one drive do not support streaming mkv
Jeremy Thomas commented
I am deeply disappointed that this support does not exist. I'm seeing that I have to move the subscription to another service.
Paul Omans commented
Microsoft is dragging their feet on .mkv support for no good reason. Please update the iOS Onedrive app with .mkv support.