In reply to Removed UserBribri:
Have a look at the Spark range from Sea to Summit. IMO some of the best specced bags on the market right now in terms of weight vs warmth. You can pretty much get two of them and cover the whole year with a very minimal weight penalty.
https://seatosummit.com/product/spark-sleeping-bag-spi/
I use the Spark SP1 for my spring/summer stuff (comfort 9c, comfort limit 5c) and paired with a decent sleeping mat I've happily used it down to 5c so far. A mate (who granted sleeps warm) has used his down to 2c whilst bivvying! It weighs 340g in a regular size.
The Spark SPIV (4) is rated to -8c comfort and weighs 880g. If you get this and either the SP2 or SP1 then you can pair them up and have a suitable sleep system for -15c to -20c comfort, whilst keeping the overall weight at 1.1-1.2KG.
It's worth pointing out that the Thermarest Oberon numbers quoted are for the comfort limit, not the comfort - this makes a big difference in terms of sleep quality. The comfort temp of the Oberon is -10c which then makes it a lot less attractive than the Spark series considering it weighs 320g more in a regular size than the SPIV despite only offering 2c more on the comfort temp - this is nearly the same weight difference as adding in the SP1 (340g) which offers far more warmth.
The main downside of the Spark series is the thinner face material meaning you need to be a little more careful with zips and sharp things. They're also somewhat spendy but not stupidly so.
Hope this helps