Definition of "oth"

They bring them wines of Greece and Araby,[*] And daintie spices fetcht from furthest Ynd,[*] To kindle heat of corage privily: And in the wine a solemne oth they bynd 35 T' observe the sacred lawes of armes, that are assynd.

1590, Edmund Spenser, Spenser's The Faerie Queene, Book I‎, published 1921