The official Oneworld sapphire extra baggage benefit is:
***Extra baggage allowance for Sapphire frequent flyers
As a Sapphire frequent flyer, the following special checked baggage privileges will apply when you fly on
oneworld airlines:
- On international or domestic itineraries using the baggage allowance "weight" system - 15 kgs in addition to the ticketed Economy Class baggage allowance.
- On international itineraries, that include domestic sectors, using the baggage allowance “piece” system – one additional bag*, weighing up to 23kgs, up to a maximum of two pieces.
- On purely domestic itineraries, using the baggage allowance "piece" system, for tickets with no ticketed baggage entitlement - the assurance of one piece of checked baggage, weighing up to 23 kgs.
QR on their website when listing the benefits of QR Gold (Oneworld Sapphire):
Extra baggage allowance
Enjoy an additional 20 kg, or one piece (depending on your route), on flights marketed and operated by Qatar Airways1.
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1Privilege Club's extra baggage allowance (in kg or piece) is based on our baggage policy, applicable on routes operated by Qatar Airways.
- When flights in the itinerary are operated by both i.e. Qatar Airways and a oneworld member e.g. LHR DOH (QR) & DOH CMB (UL); QR Privilege Club members will be entitled to the Privilege Club excess baggage allowance as Qatar Airways is the first operating carrier in your journey.
- When flights in the itinerary are operated by both i.e. a oneworld member and Qatar Airways e.g. LHR DOH (BA) & DOH DAC (QR); QR Privilege Club members will be entitled to the oneworld standard allowance as oneworld carrier is the first operating carrier in your journey.
Depending upon routing I don't think you will have any issue with at least the extra 15kg (or extra piece).
Also, see this thread (you can do a search in the thread for "sapphire" for more explanation):
Checked Baggage Allowance