Update again:
opera updated to 43.0.2442.1144 (PGO)

OOM error happened again even with all the extensions are disabled. It took longer, but the error still happens.

I agree -- youtube causes OOM error sooner.

Update:
still using VPN. I don't use Avast. Just Windows Defender.
Turned off all the extensions and the error hasn't happened yet.

I'm having the same issue. It started about a few? months back. My usage pattern hasn't changed and the symptom has started.

System= Win 10 pro 64 bit, 64GB RAM, Xeon E5 CPU, Video cards=Nvidia Quadro 600, Tesla C2050.
Opera version= 43.0.2442.991 (PGO)
Opera addons= adblock plus, flash block plus, Google search image search

My Usage pattern=

• ONLY VPN turn on. VPN is the only reason I'm using Opera.
• Single window, but multiple tabs 20-30 most of the times.

After a while, I get OUT OF MEMORY. I have to restart, or sometimes need to close some tabs and reload. If I remember correctly, a few months back when this issue started, it showed "page crashed" (just like Chrome) page, then it changed to OUT OF MEMORY error page after a few updates.

I've been changing settings, but not successful so far. Only remedy is open only several (10?) tabs. Or, if I get OOM error, close a few tabs, and reload pages -- that works sometimes, but sometimes the browser just crashes eventually. I've turned on automatic crash report, so I'm hoping Opera dev team have some idea and put out fix in near future.

Chrome and Firefox can handle multiple windows (4-5) and each window has several tabs (10-30) and they do fine. But I use different browsers for different use -- Google for certain set of sites, and FF for other types, and Opera for VPN.

I've been using Opera for a long time, even purchased when they used to sold the s/w, before Chrome came out, and Mozilla was dead, and Opera was the only competing browser against IE. I like Opera a lot, but too bad it's giving this specific error. Hope this gets fixed soon.