Can Onboard work if my OT and IT networks are separated?

Yes. In environments where the OT network (BACnet/Modbus devices) and the IT network (internet access) are physically separated, the Edge Node VM can be configured with two network adapters (Dual NIC), one bridged to the OT network for data collection and one for outbound internet connectivity to Onboard's cloud. See Troubleshooting Installation & Working with IT for setup details.