I noticed this too with an RV-5 and RC-2. I didn't think it was a big deal though. The RC-2 I took off my 1Spot and put it on it's own power supply. It was noisy on the 1Spot and doesn't automatically turn on when it's isolated. The RV-5 wasn't noisy on the 1Spot, but it did turn on. I returned it though, because in the end I hate digital effects and it was pretty useless to me. The RC-2 and RV-5 are the only BOSS pedals I've used that behave that way, and both are digital. My analog BOSS pedals are fine with my 1Spot... no noise, no turning on, nothing.
I dont know what the OS-2 is... is it digital?