The Southern Oscillation Index (SOI) is the difference between the standardized Tahiti Sea Level Pressure (SLP) and the standardized Darwin SLP measurements. They are provided by the CAC climate bulletin:
This is an important index for measuring ENSO.
The climate bulletin runs from 1951 through the present.
We also have the standardized SOI from 1881-present here. The earlier data was furnished by Judy Cole.