RUONIA (Ruble Overnight Index Average) is a weighted average interest rate on unsecured interbank overnight loans denominated in Russian rubles. It is a key benchmark for the cost of short-term money in the Russian market and serves as a reference rate for various financial instruments. An Overnight Index Swap (OIS) on RUONIA is a derivative financial instrument whereby one party (the fixed-rate payer) agrees to pay the other party interest at a pre-agreed fixed rate on a notional principal amount. In return, the second party (the floating-rate payer) pays interest based on the compounded daily RUONIA rate over the swap's term.