The spokesman said the outage, scheduled every 18 months, will involve extensive work on nuclear and non-nuclear systems at the plant.
He could not say when the unit would return to service due to competitive reasons.
The 3,090 MW Pickering station is located on the north shore of Lake Ontario in Pickering, about 30 miles (48 km) east of Toronto.
There are two 515 MW units, 1 and 4, at the A station, which entered service in 1971, and four 515 MW units, 5 to 8, at the B station, which entered service in 1983.
All of the other units were available for service.
The provincial government, which is considering the construction of new nuclear units - though not at Pickering - wants OPG to consider refurbishing the Pickering B station.
One MW powers about 1,000 homes in Ontario.
OPG owns and operates more than 22,000 MW of generating capacity and markets power to utilities in Ontario and neighboring U.S. and Canadian electricity markets.