An Overnight Index Swap (OIS) is an interest rate swap involving the exchange of an overnight (floating) interest rate for some fixed interest rate. OIS's are mainly used by banks to hedge the risk inherent in overnight interest rate fluctuations. By swapping floating/fixed interest rates, banks can insulate themselves to some extent from any adverse interest rate swings. *Keep in mind that forex markets are active 24/7 because when one major financial center like London is closing for the night, another is opening in the morning somewhere else in the world.