I have no idea what the sites in question are doing. But, you can research if they're using adaptive streaming and if there's a way to avoid it.

I don't think there's a way to make those videos buffer in full.

Sadly i agree with you. I saw some extensions that work for youtube, but not for crunchyroll or netflix. I think that we should at least choose a buffer size in opera configs

Anyway,
Thank you very much!

DASH protocol

With DASH, there's a segment size. It wouldn't make sense to have a segment size that's the whole file. So, if a site makes use of DASH, you're not going to see it fully buffer. On youtube for example, there used to be a way to use an extension to disable DASH so you could pause and buffer the whole video like it was possible before DASH. Not sure if that's still possible and whether or not you can find an extension to do that for the sites in question.

In all these services i use premium features so it shouldn't work like that.

I have no idea what the sites in question are doing. But, you can research if they're using adaptive streaming and if there's a way to avoid it.

I don't think there's a way to make those videos buffer in full.

Thank you so much for the reply.
Actually i did read some stuff today and it seems like the main cause is DASH protocol ( https://bit.ly/2hBixA2 ) not the server itself.
In all these services i use premium features so it shouldn't work like that.

If you launch a direct link to an mp4 for example in a new tab where Opera just plays it in its embedded player, you should see the whole thing buffer I think.