Due to the Star Alliance Endorsement Waiver programme, it is not necessary to obtain an endorsement from your ticketed carrier in order to fly a different airline as it usually is the case.

Instead, you should be able to present your SQ ticket to the OS desk straight away and have them change the carrier for you without any additional requirements.

But if the routing of your ticket changes (ie, VIE-SIN to VIE-FRA-SIN) then you will need to have your ticket reissued and reflecting the new itinerary, with the standard reissuing fee.

So, in your case, you would simply need to present your ticket to OS (might be advisable to make a reservation with OS before going to the airport) and they should be able to accept it with no problems. Don't forget to cancel the SQ segment, though.