No. OtO requires pressurized water in order to operate.
If you are interested in using a rain barrel, you would need something to pump the water from the barrel into the OtO device. These types of pumps usually require a high amount of power and a 110v wall plug.
The current product design does not include a pump such as this, but is a common request and likely to hit our roadmap in the future.
If you are interested in requesting this feature, please comment below or email firstname.lastname@example.org!