Glad you asked and to save on reading all the puff pieces on how it will save puppies:
A One Channel Hub is an official configuration with a single standard radio (SX12xx, LR1xxx, etc) set to one frequency & spreading factor as per the regional settings published by the LoRa Alliance. It uses the standard packet forwarder implementation.
You can register it on TTS by selecting the One Channel option for a region.
Devices are registered in a similar manner using the regions One Channel option. The firmware needs telling what frequency & SF to join at and be neutered in all ADR respects so that it doesn’t change anything.
The use case is aimed at a location with the need for half a dozen devices that don’t justify a full gateway. Something like a retail outlet that needs a few temp/humidity sensors and perhaps some power/temp monitoring of chill cabinets. If you’ve 2,450 branches as Gregs does in the UK, this makes an IoT install far more attractive in terms of capital costs.
If by chance a rogue or third-party device happens to join via the OCH, I believe the LNS will send a channel plan that means it is fixed to the single frequency/combo so it will carry on working. But as I’m still deep in the weeds of LoRaWAN Relay testing, I’ve not tried to break OCH yet so can’t be sure.