Videos are directly streaming onto my default player. Want to open in Opera so I can download it I have been using Android for the last week.

I have Opera for browsing. I  download videos from a website directly thru Opera. While downloading it asks me whether to open the video in Opera or default video player of my mobile.

Accidentally I once clicked on the "make it default" option and it was directly streamed to the video player. After this it never open in my Opera to download.

Because of this I can't download videos into my mobile. If I try to search the name of the video player I cant find it. And please help to restore as it was before.
You shouldn't need to uninstall anything. You just need to clear the default.

Go into your settings and the app manager. Find the default video player. (You'll probably have to change your view from "Downloaded" to "All" in order to find it. I expect it's called "Video" or similar.) In the settings for the app should be a button labeled "Clear defaults". Tap that and remove the existing default settings for the app.

Now when you go back into Opera and click a link it should prompt you again. Be careful about that "make it the default" button.