From Sonic Retro

OVA is an acronym that stands for Original Video Animation, a type of media originated in Japan made specially to be rented or sold on home video without prior showings on television or in theaters. Unlike in the US, "direct-to-video" has much more positive connotations in Japan, with the quality of the story and animation usually being much higher than that of an average television series. With reference to the Sonic universe, Sonic the Hedgehog: The Movie is an OVA.