All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Saying "Oh so and so" is not necessarily meant as a supplication or appeal for help or the like. It may be said for other purposes, which are deduced from the context in which it was said. Shaykh ‘Abd Ar-Rahmaan Al-Maydaani said in his book Al-Balaaghah Al-'Arabiyyah, "The vocative expression (Oh...) may be used for other than its normal usage in the Arabic language. Eloquent people and others may use it for purposes other than to call someone. These purposes may be understood from the context of one's situation or statement, such as reproach, blame, remorse, mourning, regret, allurement, appeal for help, despair, hope, remembrance, expressing grief, boredom, singling out certain addressees from others, expressing surprise, and so on." [End of quote]
People most likely use the expression "Oh mother" or "Oh father" to express astonishment and surprise without necessarily implying any sense of Shirk such as calling upon other than Allaah for help at times of distress.
Lastly, we would like to emphasize the gravity of taking the matter of Takfeer (accusing people of Kufr) lightly and not taking the criteria set by the Sharee'ah in this regard into consideration. For more benefit, please refer to fataawa 8106 and 87963.
Allaah knows best.