Hiragana is referd to as an alphabet. Each symbol represents a sound and when combined together can create words.
Hiragana is used for Japanese words where there is no kanji, particles, suffixes and endings that become attached to verbs. They can also be used for words where the kanji is not widely known or not commonly used. Hiragana can be used to write any word of Japanese origin.
When a word has a double 'o' in Hiragana, often it is written as 'ou', however this rule does not apply to every word.
A small 'tsu' character called a sokuon （っ） indicates a double consonant. The sokuon is place before the kana that is being doubled. It can't be used to double a 'n'. The Hiragana for 'n' will be added before to make it a double 'n'