Overnight Index Swap

This is a Fixed / Floating Same Currency swap
This is used to hedge interest rate risk
This swap exchanges Interest Only (no principal)

This is used for speculating on changes in the overnight interest rate.

Also known as the OIS
The third set is based on sterling overnight index swap rates, which are instruments that settle on overnight unsecured interest rates (the SONIA rate in the UK).
Tenor = 1 day

OIS curves are for nominal rates only.
OIS swaps
OIS-LIBOR basis swaps
LIBOR swaps
LIBOR-LIBOR basis swaps

Cross Currency curves
Build OIS curve to match OIS swap quotes ( used for forecast and discount)
Building Single Currency Curves first
EUR OIS curve
LIBOR 1month curve
LIBOR 3 month curve
LIBOR 6 month curve
LIBOR 12 month curve

Euro Overnight Index Average
This is a benchmark published by the European Central Bank (ECB) specifically for the EUR overnight interest rate.

GBP Sonia
EUR Eonia
JPY OIS
CHF Saron
USD Sofr

Overnight Curve