You are right. The vocative of the noun “oculus” is always “ocule” (with a short “e”). But the possessive adjective “meus” can have two forms in the vocative when it is masculine singular: “mi” and “meus”. “Meus” is a little bit more formal. “Deus” is an exceptional (or irregular) noun in that its vocative is generally “deus” and seldom “dee” (with some older writers—pre-classical— it is “dee”, though!). Put the two things together and you more often say “ô deus meus” (vocative), but “ô mi deus” is good, too! In all cases with masculine nouns in the vocative you say either “mi” or “meus”. But note that it is more usual to use vocative “meus” after the masculine noun and “mi” before it. So you usually say “ô mi domine” and “ô domine meus”.
Rectè dicis, canorcaerulecarotâclavoque. Semper “oculus” nomen vocativo casu “ocule” figuram (in “e” brevi terminantem) tenet. “Meus” autem adjectivum possessivum casu vocativo numeri singuli generis masculini duas figuras habet: “mi” atque “meus”. Aliquid formalior est figura “meus”. Inaequale nomen est “deus”, qui “deus” rarò “dee” figuram casu vocativo habet (apud nonnullos quidem “dee” vocativo scribitur! Non autem classicé sed priús!). Verbis conjunctis casu vocativo, “ô deus meus” saepiùs dicitur, sed bonum quoquè “ô mi deus”! Omnibus exemplis nominum casu vocativo, utrâ figurâ “mi” vel “meus” utamur. Cum “meus” adjectivo nomen masculinum numeri singuli succeditur, saepiùs quidem “meus” figuram dici notes; cum procedit adjectivum, “mi” figura frequentiùs se ostendit. Ergô dicere solemus ità: “ô mi domine” et “ô domine meus”.
Not so. As Swth\r says above (and Trimalchio), “Meus -a -um” is a Possessive Adjective, and is declined like any First and Second declension adjective (with the exception of the masculine singular vocative). So in the Masculine case it is MEUS (Nom), MI or MEUS (Voc), MEUM (Acc), MEI (Gen), MEO (Dat), MEO (Abl).
Ego (me mei mihi me) is the Personal Pronoun, declined EGO (Nom), ME (Acc), MEI (Gen), MIHI (Dat), ME (Abl).
Minimé. Ut suprà dicit Swth\r (et Trimalchio) adjectivum possessivum est “meus -a -um”, quod, vocativus casus separatim, sicut adjectivum primae secundaeque declinationis inflectitur.
Strictly speaking the Personal Pronoun “EGO” does not have a vocative. If you want to write a pychoanalytical drama where you have a character called “EGO” (representing the Freudian Ego, of course), keep his name indeclinable and address him vocatively as “Ô Ego”.
Astrictè dicere, “ego” Pronomen Personale casum vocativum non tenet. Si te dramatem psychoanalyticum scribere velis in quo in proscaenio appareat persona “Ego” nomine (quod ideam Freudianam demonstrat, certé), facias ut nomem indeclinabile sit et is “Ô Ego” vocativo casu appelletur.
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