Hi Linus,
If I understand correctly, you have a source linked to a osc input value and you want it to emit power at 100% when the osc input is 0% and 0% when the osc input is 100% (inverted).
I see two ways of doing it. The first way is to edit the source's power formula and use something like: 100-osc(0). I'm using osc(0), but you need to use the index of your osc input.
Another way is to do the inversion at the attribute level. You can select the intensity attributes targeted by your source, and check the Inverted box.