Correct, southern Oscillation Index.
It's a measure of the SLP pressure difference between Tahiti and Darwin, Australia which can in turn be used in conjunction with ENSO as one metric to assess how coupled the atmosphere/ocean are. SOI alone doesn't hold significance or weight on the pattern (at least in terms of influences here) but it can enhance the effects an ENSO event will have on the pattern across the PAC which could have some downstream bearing here.
SOI (along with Multivariate ENSO Index (MEI), and Relative Oceanic Nino Index (RONI)) can provide a more accurate assessment of the true strength of an ENSO event versus ONI alone.