The SO in ENSO refers to the Southern Oscillation,which are the changes in the atmospheric circulation (specifically the sea level pressure across the Pacific ocean) that accompanies the changes in ocean temperature and makes ENSO a coupled ocean-atmosphere phenomena.  For further details on the origin of the acronym, check out the very first ENSO blog post from May 2014:

http://www.climate.gov/news-features/blogs/enso/what-el-ni%C3%B1o%E2%80…