Goal-kicking full-back Cashmore, 32, has two caps for the All Blacks and played for the Bay of Plenty against the British & Irish Lions on their 2005 tour. He has agreed a two-year deal with the Welsh regional side.
Karauna is 30 and a utility back who can play at wing, centre and full-back and was recently chosen for the New Zealand Sevens squad.
Ospreys rugby manager Derwyn Jones said: "Adrian has pace and is an accomplished goal-kicker and all-round footballer who would have won a sackful of All Blacks caps were it not for Christian Cullen.
"It is widely recognised in New Zealand that he would have had 10 years of international rugby were it not for Cullen and his quality and ability is exceptional.
"He is an excellent professional and I am sure our younger players will learn a lot from him."