The OneWorld site shows differing baggage limits, depending on where the flight originates. The most liberal:

Travel to/ from/ via USA, Canada, Mexico, Central America, Caribbean, South America & Australia (except travel wholly between Europe and South America and Spain & the Caribbean and travel wholly between South and Central America).

Two (2) pieces 32kg (70lbs) each piece with total dimensions 158cm (62ins) allowed for each piece

Then, it lists:

All other International routes (including travel wholly between Europe and South America, Spain & the Caribbean and travel wholly between South and Central America

First 40kg (88lbs)
Business 30kg (66lbs)
Economy 20kg (44lbs)
British Airways Premium Economy and Economy 23kg (51lbs)

So, I guess you need to look at your itinerary and chose the most restrictive luggage limit for any one flight sector.