I asked a Romanian friend about a word I'd encountered frequently and could not find in a dictionary "o."
"O" has other grammatical functions, including a conditional and various idiomatic usages. It's definitely a word.
"It's not a word," he said.
He offered no explanation for the fact that it's common. Puzzled, I dropped the matter knowing that I'd find my answer later.
"O" is a word. It is one of two indefinite articles a cognate of a(n.) [In the English language, "a" and "an" are the same word except that they're pronounced and spelled differently according to whether the following word begins with a consonant or a vowel sound. The distinction serves to ease pronunciation, only.]
Every noun in Romanian uses as its indefinite article either "o" or "un," and one or the other is always correct for that noun. The use of "o" versus "un" is based upon gender not upon masculine vs. feminine, but upon the matter of whether or not the noun has a gender. It's complicated, and confusing to a native English speaker, for whom gendering of nouns is unfamiliar.
I've never corrected my friend. I think of the interaction as just another example of the uselessness of the bilingual native speaker. There's nothing you can do about it.
As with the two definite articles in Dutch (Nederlands,) a student of the language has a choice learn by rote or speak incorrectly a large percentage of the time.