Shortened form of "Ohayou gozaimasu," the Japanese salutation equivalent to "Good morning." It is a cute way of saying hello.
"Oha" was made popular by a Japanese celebrity, Shingo Katori, on the TV show "Saturday SMAP." He uses it as his catchphrase, and used it in his song "Shingo Mama no Oha Rock."
A hand gesture usually accompanies an "Oha!" greeting. On the first syllable "oh", hold up your hands, palms out, with your index fingers touching your thumbs (like the American "O.K." gesture). On the second syllable "ha," straighten out your index fingers and thumbs, and push your hands forward.
Oha! How are you doing this morning?