O Mistress Mine is an Elizabethan song which appears in Shakespeare's play Twelfth Night.
There is an instrumental piece entitled O Mistress Mine by Shakespeare's contemporary Thomas Morley which appeared in 1599.
Whether or not this was the case, Shakespearean scholars think that Morley's publication predates the first performance of the play.
Morley held a printing patent (a type of copyright protection) and published the tune in First Booke of Consort Lessons (1599).
O, stay and hear; your true-love's coming, That can sing both high and low: Trip no further, pretty sweeting; Journeys end in lovers' meeting, Every wise man's son doth know.