There is a command-line switch to make it run in single-process mode ... that will obviously make Opera slower, but put less strain on your system.

A Google search lands me here, where it says ...

Thankfully Chromium still supports the flag and you can invoke it by using --single-process (note the double dash).

Having said that, I have a Toshiba laptop with 2 GB RAM and other specs similar to yours where Opera is not a serious problem, but I tend to have relatively few tabs open at any one time.