I like Oneworld as it is the only alliance that gives mid-tier members the opportunity to use their business class lounges. Star Alliance and Skyteam both require top-tier before granting that benefit.
There are currently a couple of ways you can get Oneworld Sapphire without flying the full amount normally required.
American Airlines offers an elite status challenge whereby you call in (telephone numbers here) a pay a fee to request it. There are two levels, Gold (Oneworld Ruby) and Platinum (Oneworld Sapphire), but I will only refer to the Platinum in this post.
The fee is $200 to sign up and to successfully complete the challenge, you need to earn 12,500 Elite Qualifying Miles in 90 days. The miles must be earned from flights marketed and operated by AA, British Airways, Finnair, Iberia, Japan Airlines or Qantas.
Note that it is significantly easier to complete the challenge if you are flying on flights marketed by AA as their earning rates are more favourable – full-fare first class or business class earn 3 EQMs per mile flown, 2 EQMs for discounted first and business, 1.5 EQMs for full-fare economy and 1 EQM for discount economy. If you are flying on Oneworld partner airlines you should check the earnings pages here.