Mengamanhttps://jdslabs.com/product/15vac-power-adapter/
I'm 99% sure that's basically what you're looking for - I don't own drops O2 but I assume they use the same barrel connector sizes. Sadly it isn't cheap. At least you have the assurance that this is what jds labs uses for their version of the O2. On the plus side you'll be getting a premium adapter compared to Drops.
If you don't go through JDS labs looking for a good deal elsewhere (ebay/amazon), be careful that you don't get a 15V DC adapter. I've seen listing say they're VAC in one spot and VDC at the bottom of the page. You also need the barrel to be 2.1 x 5.5 x 11.0 mm and that's even rarer to be specified on ebay/amazon (granted it's like the most common size). Mouser and Digikey also sell these kinds of products but from memory it was like the same price or more.
You could get something that's 14Vac and be just fine as long as the current is like above 250mA (more the better for current but diminishing returns - usually cost more the higher the current rating). I wouldn't go above 15Vac for the voltage. Going much below 13Vac probably a bad idea.