The actual meaning of "O ΩΝ" is not "I AM" (first person) but rather "THE ONE WHO IS"(third person).
Almost. O ΩΝ is actually a participle (a masculine nominative singular present active participle) and, thus, being a verbal adjective, does not have a person (third or otherwise).
However, you are indeed correct that it can be translated as "THE ONE WHO IS". Another good translation would be "THE EXISTING ONE". The latter is a bit more literal, since it more fully conveys the participial nature of the Greek.
The translators of the Septuagint decided to replace the Hebrew "I AM" with O ΩΝ -- not a literal translation, but one that captures the essential meaning and conforms with the prevalent Hellenistic philosophical language/understanding of what true Divinity must be like, i.e. that true Being must, by definition, be eternal and unchangeable (as Parmenides taught). Thus, the translators of the Septuagint were making it clear that this God to whom Moses was speaking was indeed the true God.