Thanks, I did not know about Task manager in Opera but unfortunately it does not show the disk activity.
Anyway, according to Windows Task manager the Opera itself produces the disk activity.

I don't have any extensions, at least registered on extensions page. I believe I have installed only Adobe Flash, but I cannot see it there.

I don't think so. I don't visit 80Gb pages per a week. This should be streamed video at least...
I just have Opera always opened at the background. There are a lot of opened pages, I believed they automatically reload
ofter but I have only 300 MB downloaded in 11 days, according to wifi info. I have restarted Opera with the newwest update, just about half an hour ago and it has already written 1GB and keeps writing about 30-50 Kb. each second.

Opera (latest version) writes huge amount of bytes on HDD, several days of using (laptop hybernated over the night) made about 80 Gb written to HDD. With SSD it is a noticeable problem because of a limited resource. Anyway, I cannot understand, what is that browser needs to write on HDD of such size??
And, how to fix this?