Happened for first time to me today. 10 tabs open (About low average for me). No videos going. 4 of the tabs received OOM errors apparently simultaneously. Other 6 were fine. The tab I was on was nextwharehouse.com.
OS: Win7 Ultimate latest updates and patches
CPU: i7-5820k no overclock
GPU: EVGA GTX 660 no overclock
Mem: Crucial Ballistix Sport LT Red 64GB DDR4-2400 no overclock
MB: ASROCK X99 Extreme4
Ad Block Plus
Microsoft Security Essentials
No VPN at the time
Memory usage indicated 31%
My system generally idles through everything I do with it and with 64GB of gaming grade memory, not sure how Opera could have detected, let alone produced an OOM condition.