From
http://www.oneworld.com/ffp/sapphire-privileges
Extra baggage allowance*
As a Sapphire frequent flyer the following special baggage privileges will apply when you fly on oneworld airlines.
On international or domestic itineraries using the baggage allowance "weight" system you receive 15 kgs in addition to the ticketed Economy allowance.
On international itineraries (including domestic sectors flown as part of international itineraries) using the baggage allowance "piece" system, you will receive a second bag, weighing up to 23 kgs, when the ticketed allowance is one bag.
On domestic itineraries only with flights using the baggage allowance "piece" system, you are guaranteed at least one piece of checked baggage, weighing up to 23 kgs, even when the regular ticketed allowance has no free allowance.
Customers travelling in First and Business Class cabins will in most cases receive an even greater free checked baggage allowance.
*This benefit is not available on British Airways 'Hand Baggage Only' fares on select short-haul routes.
*For domestic routes operated by LAN Airlines, Sapphire cardholders may check in up to two bags with a combined weight not exceeding 23 kgs.
So that's almost the same as the OneWorld Emerald allowance in practice. BA HBO excluded as one might expect, and an additional LAN restriction also (which OW Emerald does not have).
That's a very nice benefit indeed for a fairly easy-to-achieve level of status.