Είμαι εντάξει. Είμαι μια χαρά. Είμαι ΟΚ. Όλα μού πηγαίνουν πρίμα. (Way too colloquial!!!). Όλα καλά/Όλα ήρεμα. (It means that everything is fine, quite in one's life). All the above mentioned expressions are quite colloquial and you should make sure that are more easily used among young people or among people who know each other well, no matter the age difference. For example, among uni students or between you and an aunt you love a lot and feel close to.