Before the creation of OASTH in 1957, public transport in Thessaloniki was initially covered by an extensive tram network opened in the late 19th century.

[2] OASTH was founded by Presidential Decree 3721 and aimed at replacing the city's tram network as sole provider of public transport.

In 1979 OASTH expands, taking over several routes previously operated by KTEL in the suburbs of Thessaloniki and also takes over the sea services operating by small boats during summer months and are replaced by new OASTH bus routes.

The Greek government in 2017, through the passing of a law, proceeded to nationalize the company in 2017, citing as a cause the poor service of the citizens, the corruption and the complaints for embezzlement of public money by the private shareholders.

The current fleet of OASTh is 622 (347 in operation, 249 modular, 22 small vans and 4 specially configured for movement of disabled people) consists of the following bus types: