@piesekchlebek The person that builds the libffmpeg.so you use would have to build in support for it. I don't think any of the people provide libffmpeg.so by itself have one that's has support for h.265. You'd have to build it yourself.
However, there are some Chromium Linux builds at https://chromium.woolyss.com/ (down the page) that have h.265 support. See the ones with all-codecs+. Nothing like that for Opera as far as I know. I guess you could extract libffmpeg.so from the package and try it, if you can find a Chromium 119 all-codecs+ package. Or, you could ask one of the developers of those builds to see if they'll provide you with an h.265 libffmpeg.so that's compatible with Chromium 119. Don't know if they'll do it or not, but you can ask.
There might be some hoops you'll have to go through and flags you have to mess with and drivers you have to use to make it work. Not sure. I would try to get it h.265 to work in the Chromium build first before trying its libffmpeg.so with Opera.