What Is Open Interest (OI) in Stock Market?

Introduction

If you have ever looked at an Option Chain or Futures Market, you have probably noticed a column called Open Interest (OI). Many beginner traders believe that Open Interest simply means trading volume, but that is not true.

Open Interest is one of the most powerful indicators used by professional traders to understand market participation, identify trends, and confirm whether money is entering or leaving the market.

Whether you trade NIFTY, BANK NIFTY, Stock Futures, or Stock Options, understanding Open Interest can significantly improve your decision-making process.

In this complete beginner's guide by Sathi Capital, you'll learn:

What Open Interest means
How Open Interest works
Difference between OI and Volume
Why professional traders watch OI every day
How beginners can use OI effectively

By the end of this guide, you'll understand Open Interest like a professional trader.

What Is Open Interest (OI)?

Open Interest (OI) is the total number of active derivative contracts (Futures or Options) that are currently open and have not yet been closed, exercised, or expired.

Simply put,

Open Interest represents the total number of outstanding contracts in the market.

Every time a buyer and a seller create a new contract, Open Interest increases.

When an existing contract is closed, Open Interest decreases.

Unlike stock trading, Open Interest only exists in derivatives, such as:

Futures Contracts
Call Options
Put Options

It does not apply to normal equity shares.

Simple Example

Imagine two traders.

Trader A buys one NIFTY Call Option.

Trader B sells one NIFTY Call Option.

Since this is a new contract, Open Interest becomes:

OI = 1

Now another trader enters.

Trader C buys one new contract.

Trader D sells one new contract.

Open Interest becomes:

OI = 2

Now suppose Trader A closes his position.

Trader B also exits.

Open Interest decreases back to:

OI = 1

This is how Open Interest changes every day.

How Does Open Interest Work?

Many beginners think buying increases OI.

Actually, that is only partially correct.

Open Interest changes only when new contracts are created or existing contracts are closed.

There are four possible situations.

Scenario 1

Buyer enters

Seller enters

Result:

✅ New Contract Created

Open Interest Increases

Scenario 2

Buyer exits

Seller exits

Result:

Existing contract removed

Open Interest Decreases

Scenario 3

Old Buyer sells to New Buyer

Result:

Total contracts remain same.

Open Interest Does Not Change

Scenario 4

Old Seller transfers to New Seller

Again,

No new contract is created.

Open Interest remains unchanged.

Why Is Open Interest Important?

Open Interest tells us whether fresh money is entering the market or money is leaving the market.

This information cannot be obtained from price alone.

Professional traders use OI to answer questions like:

Is today's move genuine?
Is the trend strong?
Are institutions building positions?
Is a breakout likely to continue?
Is short covering happening?
Is long unwinding happening?

That's why almost every professional derivatives trader monitors OI throughout the trading session.

Open Interest vs Volume

Many beginners confuse these two terms.

Although both are important, they measure completely different things.

Open Interest	Volume
Total active contracts	Total contracts traded today
Measures market participation	Measures trading activity
Indicates fresh money entering or leaving	Indicates how much trading happened
Updates as contracts open/close	Resets every trading day
Used for trend confirmation	Used for liquidity analysis

Example

Suppose today's trading volume is:

50,000 contracts.

But Open Interest is:

7,50,000 contracts.

This means:

Only 50,000 contracts traded today,

while 7.5 lakh contracts are still active in the market.

That is why Open Interest gives a much bigger picture than Volume alone.

Why Professional Traders Use Open Interest

Institutional traders rarely rely only on price.

Instead, they combine:

Price Action
Volume
Open Interest
Option Chain
Support & Resistance

This combination helps them understand whether a market move is backed by real participation.

For example:

Price rising + OI rising often suggests fresh buying interest.
Price falling + OI rising can indicate fresh short positions.
Price rising + OI falling may point to short covering rather than new buying.
Price falling + OI falling often signals long positions being unwound.

In the next part, we'll explore these market conditions in detail and learn how to interpret them like professional traders.

Key Takeaways

Open Interest measures the total number of active Futures and Options contracts.
OI exists only in the derivatives market, not in equity shares.
Rising OI generally indicates increasing market participation.
Falling OI often suggests traders are closing positions.
Open Interest and Volume are different metrics and should not be confused.
Combining Price, Volume, and OI provides stronger trading insights than using any single indicator.

Internal Links :

What Is Option Chain? Complete Guide (2026)
What Is Futures & Options (F&O)?
What Is Technical Analysis?
Support & Resistance Explained
Price Action Trading Guide
Risk Management in Trading

Part 2 - Understanding Open Interest in Futures & Options

Open Interest in Futures & Options

Open Interest (OI) is available only in the derivatives market, which includes:

Futures Contracts
Call Options (CE)
Put Options (PE)

Whenever a new buyer and seller create a derivatives contract, Open Interest increases. When both parties close their positions, Open Interest decreases.

Unlike equity shares, Open Interest tells traders how many contracts are still active in the market.

Open Interest in Futures

Suppose two traders enter a NIFTY Futures contract.

Trader A buys 1 Futures Contract.
Trader B sells 1 Futures Contract.

A new contract is created.

Result

Open Interest = 1

Now two more traders create another Futures contract.

Result

Open Interest = 2

If one Futures contract is closed,

Result

Open Interest decreases to 1

This helps traders understand whether fresh money is entering or exiting the futures market.

Open Interest in Options

Options work exactly the same way.

Whenever a new buyer purchases an option from a new seller,

A fresh option contract is created.

Example

Trader A buys one Call Option.

Trader B sells one Call Option.

Open Interest increases.

Later,

If both traders square off their positions,

Open Interest decreases.

This is why every Option Chain displays Open Interest separately for:

Call Options (CE)
Put Options (PE)

What Is Option Chain?

An Option Chain is a table showing all available Call (CE) and Put (PE) option contracts for different strike prices of an underlying asset, such as NIFTY, BANK NIFTY, or individual stocks.

It provides traders with important market information, including:

Strike Price
Open Interest (OI)
Change in OI
Volume
Last Traded Price (LTP)
Bid & Ask Prices
Implied Volatility (IV)

Professional traders use the Option Chain every day to understand market sentiment and identify important support and resistance levels.

How to Read Open Interest in an Option Chain

Imagine the following simplified Option Chain:

Strike Price	Call OI	Put OI
24,400	5,20,000	1,80,000
24,500	8,50,000	2,10,000
24,600	10,40,000	3,00,000
24,700	4,10,000	6,20,000
24,800	2,30,000	9,80,000

Interpretation

Highest Call OI = 24,600
Highest Put OI = 24,800

This generally suggests:

24,600 may act as a strong Resistance.
24,800 may act as a strong Support.

Important: Open Interest alone should never be used to predict the market. Always confirm with price action, volume, and market context.

Why Does Open Interest Increase?

Open Interest increases whenever new contracts are created.

This usually indicates:

Fresh market participation
New positions being built
Institutions becoming active
Higher liquidity
Stronger conviction among traders

A rising OI often signals that traders are willing to commit fresh capital to the market.

Why Does Open Interest Decrease?

Open Interest decreases when existing contracts are closed.

This may indicate:

Profit booking
Position unwinding
Reduced market participation
Expiry-related exits
Traders waiting for new opportunities

A falling OI shows that contracts are being removed from the market.

Four Important Open Interest Signals

Understanding the relationship between Price and Open Interest is one of the most valuable skills for derivative traders.

1. Long Build-up (Bullish)

Conditions

✅ Price ↑

✅ Open Interest ↑