Alan wrote:'ー' ... is only used in Katakana. In hiragana, you just repeat the vowel again. However double 'o' usually becomes 'ou' to confuse matters a bit more.
This isn't exactly true... It is acceptable to use the dash in many native Japanese (i.e. hiragana) words as well. For example, 大きい=おーきい and 応接=おーせつ. You can't, however, use the dash in words such as 小売り (こうり), because the 'o' and the 'u' belong to different kanji. Whereas the 'o' in 講師 (こうし, こーし) is pronounced long (because the 'o' and 'u' belong to the same kanji), the 'o' in 子牛 (こうし) is pronounced short. There is no pause between syllables, but it should sound more like 'ko-ushi' than 'kōshi'. Thus, the dash cannot be used.
And it's actually not that difficult to tell when 〜お and 〜う are used to write long 'o'. 〜お is used mainly in readings of Japanese origin, and 〜う is used mainly in readings of Chinese origin. That means kun-yomi and on-yomi respectively. So 大きい is おおきい, 公 is おおやけ, and 多い is おおい (kun-yomi words). 応接 is おうせつ, 欧州 is おうしゅう, and 横断 is おうだん (on-yomi words). There just happen to be far more long 'o' sounds in on-yomis than in kun-yomis.