Saphire benefits
 

As a OneWorld Saphire, do I get extra baggage allowances for all One World airlines or for just to the airline who has issued the card?

You do not -- so become sapphire on the airline where you want that benefit. The baggage policy is described at http://www.oneworld.com/partner/baggagepolicy.cfm

Note that RTW tickets use the piece system giving you much more weight than other flights (even your sapphire bonus). Many airlines are now enforcing the baggage limits and charging for over-weight bags.