Johnson became mayor in 2008. Oyster cards were introduced in 2003.
The most generous interpretation is that Johnson was referring to schemes introduced while he was mayor aimed at increasing the use of Oyster cards, although he gave no indication he was doing so. He could have been referring to the fact that contactless payment was introduced on London buses in 2012. But Oyster cards had been accepted on buses since before he became mayor.
Johnson’s statement is misleading.