Austrian Airlines and Slovak Airlines are code-sharing on the BRU-BTS route. The 0940 OS flight was scheduled to be operated by a B737 but yesterday it was only an F70. Together with the canceled 6Q flights, it seems that this route isn't doing very well.
Add to that the fact that Bratislava could be called Vienna East, and the competition is even higher with SN BA and the OS flights to VIE in the picture!