"Friends" question, please help :)?

i may be wrong but i think the sentence 'buying a vowel' is popularised from the wheel of fortune game where you're supposed to spin the wheel and guess on a phrase with letters. and for vowels, you have to buy them from the money you've got from spinning the wheel. the rest of what she said doesn't really mean anything special. just means that she's buying an "O " for the phrase O(h) My God.