Is it that we were low enough to get back to a safe altitude quickly enough without the need for O2?
Short answer : Yes.
Long answer : Yes, the saab is pressurised and yes, you will generally be anywhere from F090 up to F250 depending on the length of flight.
The simple answer is that the regulations do not require outlets for all pax on the saab. The time taken to descend in an emergency to F100 or below in the saab is much less than the time of useful conciousness at saab cruising levels. The oxy outlets in the cabin are primarily for use
after a decompression, in case a passenger has difficulty breathing even when at F100.
a Metroliner - which would have been more appropriate for the Orkney run
Eh, complete rubbish. If that were the case you'd still be flying on the Shorts. For a start, the metroliner only seats 19 people, the saab has 34 (usually) and that still isn't enough!