It's a slight alteration on the japanese exclamation "ara", which confers to "
oh?" or "
ah?"
Most cases, such as on the
Rurouni Kenshin TV DVDs, it's left untranslated since, and your professor is right, it is just a sound, easily understood in that context as just an outburst
exclamation.
This, of course, is coming
not from my personal knowlege, but after having read the liner notes on the first
DVD of the
Rurouni Kenshin TV Series.
NB: Kenshin's "oro" is written only in
katakana, to distiguish it from the phrase "oro":
悪露 (oro)
lochia, post-natal
vaginal discharge
The kanji, incidentally, mean "
evil" and "
dew," respectively
>_<