First introduced in 1990, the Mercedes Benz O405 formed the bulk of SBS's air-conditioned fleet throughout the 90s. 200 units were bodied by Walter Alexander under the PS series, and were fitted with Fuji or Webasto air-conditioning.

SBS Transit O405s are of Mk I type - They were fitted with the naturally aspirated OM447h engine (11,967cc) and W3E 110/2.2R 3-speed automatic transmission (a few units however were replaced with ZF or Voith automatic). Originally fitted with grey/blue/grey leather seating, the seat covers and air-con ducts were refurbished in the early 2000s.

The fleet of Walter Alexander O405s were fully retired by 2010. Upon retirement from SBS Transit, selected units were sold second-hand to Bangkok, Thailand, where they were re-bodied, converted to Compressed Natural Gas (CNG) operation and deployed on its air-con routes.