OT is short for Operational Technology, it is the industrial production version of IT (Information Technology). Companies tend to split their networks through a firewall to keep their IT network and OT network segregated ensuring production is further away from the internet and harder to be victim of a cyber attack.
OT networks are typically futher split up into VLANS based on the Purdue Model to bring even more security to the operational devices. The validity of this approach recently is under constant question.