OASIS, or the Organization for the Advancement of Structured Information Standards, is an international non-profit organisation that works with standardization to advance electronic commerce. In other words, they make standards that live on top of standards created by other standardization organisations like W3C and IETF. At the moment the most well known projects hosted with OASIS are DocBook and ebXML, but they also do a lot of work on XML and XSLT conformance.
OASIS is an open member organisation, as opposed to the W3C, and both individuals and companies can become members. If working with standardizing structured data exchange sounds like something for you I'd recommend that you join.
The official OASIS web site is at http://www.oasis-open.org/ and not at the more obvious one...