It depends on whether you're a fan of split-testing or Internet marketing forums.
If you're into split-testing, WSO stands for Website Optimizer, the one put out by Google.
If you're into Internet marketing forums, WSO stands for Warrior Special Offer.
When somebody says WSO (which happened to me yesterday), you must know from what background they're coming from. In this case, the guy I was talking to was into split-testing, so I knew what he was referring to.
Anyway, just a random piece of information that may come in handy when you're talking to prospects or other online marketers.