I had this issue yesterday. The computer said out of memory on both browsers and then the computer shut down and applied updates to windows 7. Not sure if updates had anything to do with it but i'm pretty sure it was twitch related because thats what I was watching and I have had this error before watching Twitch streams.

I was watching Twitch with 2 streams one on each monitor. Also I was closing and opening a 3rd stream. I have had this issue in the past with just one stream but for today It mainly was two streams one on each monitor.

I just upgraded my memory from 4 GB to 16 GB

Here are my specs

CPU: I5 2500k
RAM: 16GB RAM
OS: Win 7
Opera version: 43.0
Video card: GTX 750ti 2GB

Could the problem be with overbuffering? Maybe Opera is not allocating the memory correctly with Twitch? I'm not exactly sure how streams work are they buffered in RAM or the HD? Im assuming they get buffered to the HD. Well just wanted to report this issue. Thanks!