The term changes back and forth but OC and CWN are On Call and Call When Needed, which designate a type of contract. All tankers operate under a contract with the federal government or a state; the open contract is CWN, and federal CWN aircraft are available for use or release at any given time. A CWN aircraft has no guarantee of use; it's entirely up to the government when it starts, when it's released, and there's no way for the crew to now when they'll be called out or when they'll go home.
The other kind of contract is EU or Exclusive Use; these are contracts with designated start and end dates, and guarantee a certain amount of time to the government. Typical exclusive use contracts are anywhere from 90 to 120 days or more. The aircraft may have a particular designated base to start, but the if the contract is federal, the aircraft is classified as a "national asset" and can (and does) go anywhere in the country on a moment's notice.
Exclusive Use contracts have a known period of guaranteed availability and pay, but are also always available for pre-season and post-season use, which makes them more like CWN aircraft.
Aircraft and crews are paid for daily availability, or a daily rate for being available up to 14 hours a day. Depending on the aircraft and contract, typically hours over 9 are also paid a small additional override, up to 14 hours. Some contracts pay an hourly flight rate, while others have a monthly salary paid to the crews, and no hourly rate (designed to eliminate the incentive to fly, in the interest of making safe go/no-go decisions). The aircraft earns daily and hourly rates, regardless of what the crew is paid. Some contracts include per diem, others don't, though most all operators pay their personnel per diem whether the government pays the contractor, or not.