AO does still operate SIN-DPS but AO is not an OW carrier so isn't , I believe, valid for LONE fares
According to the BA document ONE fares validity
First/Business/Economy travel valid for Round-the-World (RTW) travel incl open jaw trips via AA/AY/BA/CX/EI/IB/LA/QF operated flights.
Oddly though, the same document lists AO as valid for GLOB tickets
Dave