What it means
The mechanics are simple. Two parties agree a notional amount purely as a reference, one leg pays a fixed rate for the term, and the other pays the compounded average of the daily overnight index, such as SOFR in the United States or SONIA in the United Kingdom.
At maturity, or periodically, only the net difference is exchanged. The contract matters far beyond trading desks because the fixed rate on an OIS is a clean market forecast of average central bank policy over the term.
A one-year OIS quoted 0.75 percentage points above the current overnight rate is telling you the market expects roughly three quarter-point rate rises over the year. Corporate treasurers use OIS to convert floating exposure into a known cost, or the reverse, without borrowing or lending any principal.
A company paying floating interest on a loan can pay fixed on an OIS and receive floating, which offsets its exposure and leaves it with a predictable interest bill. The OIS curve is also the standard discount curve for valuing collateralised derivatives, because cash collateral typically earns the overnight rate.
That technical role means OIS quietly sits underneath the valuation of a large share of the derivatives market. One widely watched indicator is the spread between a term interbank rate and the equivalent OIS rate.
A wide spread signals that lenders are worried about bank credit and funding, and it is followed closely as a stress gauge.
In practice
Real-world examples.
Example
A bank funds itself overnight but has lent at a fixed rate for two years. It enters an OIS receiving fixed and paying the overnight index, which matches its funding cost to its lending income.
Example
An asset manager wants exposure to an expected series of rate cuts without buying bonds. Receiving fixed on a two-year OIS gives that view directly, with no principal outlay and no credit exposure to an issuer.
Example
A risk team tracks the spread between three-month term rates and three-month OIS. When it widens from 0.15% to 0.55% in a fortnight, the team treats it as an early warning of funding stress and reviews its counterparty limits.
Think of it
“OIS swaps fixed for overnight floating rates-used to price rate expectations.
Formula
Calculation
Net settlement = notional x (fixed rate - realised compounded overnight average) x days / 360.
A treasurer enters a three-month OIS on a notional of $50,000,000, paying a fixed 4.00% and receiving the compounded overnight index, with 90 days in the period on a 360-day basis. Over the quarter the overnight rate averages 3.60% after compounding, because the central bank cut earlier than expected.
The rate difference is 4.00% - 3.60% = 0.40%, and the day-count fraction is 90 / 360 = 0.25. Net payment = $50,000,000 x 0.0040 x 0.25 = $50,000. The treasurer pays $50,000 to the counterparty, and no principal moves at any point.
Had the average instead been 4.30%, the difference would have been 4.30% - 4.00% = 0.30%, and the treasurer would have received $50,000,000 x 0.0030 x 0.25 = $37,500.Case study
Seen in the real world.
The following is an illustrative and fictional example. Larksfield Utilities, an invented regional water company, had $300,000,000 of floating-rate debt priced off the overnight index and a regulator that set its allowed revenue years in advance. Rising rates threatened to push interest costs above the amount its tariffs were designed to recover.
Rather than refinance the debt, which would have cost about $4,000,000 in fees in this fictional scenario, the treasury team entered a series of OIS contracts paying fixed at an average of 3.80% on $240,000,000 of the exposure. The overnight index went on to average 4.60% over the following two years, and the swaps paid Larksfield roughly $1,900,000 a year, which offset most of the increase in its loan interest.
The illustrative point is that the swap did not remove the debt or change the lender; it simply converted the shape of the interest cost to match the shape of the regulated revenue.
Watch out
Common mistakes.
- Believing the notional amount is at risk, when only the net interest difference is ever exchanged.
- Reading the fixed OIS rate as today's overnight rate, when it is the market's expected average rate over the whole term.
- Treating an OIS as free of credit risk entirely, when there is still counterparty exposure on the net amounts owed.
Questions
People also ask.
Why is OIS considered close to risk-free?
Because the overnight rate reflects lending for a single day against a policy-anchored benchmark, so it carries almost no term or credit premium.
What does a widening interbank-to-OIS spread mean?
It signals that banks are charging each other more for term funding relative to the risk-free path, which usually reflects credit or liquidity stress.
Can a non-financial company use an OIS?
Yes, treasurers of larger corporates use them regularly to manage floating-rate exposure, subject to credit approval and collateral arrangements with their bank.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%