Pilots in commercial operations must conduct two check flights a year-a license proficiency check (annually) and an OPC.
In addition to that, a 12 monthly Line check needs to be carried out as well.
While the OPC and License proficiency check can be done interchangeably, the Line check is something different.
Most operators have an approved program to qualify pilots to fly in either seat, at a minimum, a certain number of landings and an auto rotation must be carried out from the other seat (usually it is required to carry out one opc from the other seat to be qualified).
Thereafter, each OPC should be carried out from the opposite seat.
Does that help?
Edit:
Just seeing that this might be misunderstood.
Of course there must be an opc every 6 months, of which one can be the License Proficiency check, and one line check within 12 month.. Hope that clarifies what I failed to explain better before..