The term odachi (kanji: 大太刀) means "large tachi sword" in English. It is written おおだち in hiragana. It may also be rendered in English as oodachi or ōdachi. It's pronounced roughly like "oh dah chee".
Roughly speaking, a tachi sword longer than 86 cm qualifies as an odachi. Also known as nodachi, 野太刀.
The kanji 大, by itself, can be pronounced o, dai, or tai. It can be found, for example, in the Japanese words for university (daigaku, 大学, だいがく) and embassy (taishikan, 大使館, たいしかん).