o-(oh)see note 1
1: the second 'o' shown here is a grammatical particle known as an object marker, see the help page on particles
kc says: while not all these characters start with an a, all characters in the 'a' category are vowel characters. this category is used in both hiragana and katakana sections.
kc says: the hiragana alphabet is the primary native japanese alphabet, it can be used to phonetically write all native japanese words. for more information about the different alphabets see the main help page