(O.S.) Increase Mather, American minister and author (died 1723)

Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923.

Increase Mather was a New England Puritan clergyman who served as the sixth president of Harvard College from 1685 to 1701. During his tenure, which coincided with the notorious Salem witch trials, he was influential in the administration of the Massachusetts Bay Colony.