Yes "ollaimh" (professor) works the same as others beginning with a vowel e.g. "athair" (father) etc. in not needing the "a" before it, and thus doesn't change in it's spelling/pronounciation like "caraid"/"a charaid" (friend) and others beginning with a consonant do :)
Because you address male (person or object) that doesn't have "i" as the last vowel, you need to add "i" after that last vowel. You don't need "a" at the front, because the word starts with a vowel, but there is still something different.
Gaelic uses a formal/informal distinction, unlike Irish. It's appropriate to use the formal form when addressing a professor. If you know other languages with formal/informal, those rules will probably put you in roughly the right ballpark for when to use either form.