There's no way to convert between the weekly OISST value and the seasonal ONI value, since they use different underlying data sets.  For the past few months, the data set used for the ONI (ERSST) has been running a bit more than 0.5C cooler than the OISST.  You can read more about the different data sets in this blog post:

https://www.climate.gov/news-features/blogs/enso/exactly-same-completel…