Order Book
The live, ranked ledger of every pending buy and sell order — the engine that determines where trades execute and at what price.
Definition
The order book is the electronic record maintained by an exchange listing all outstanding, unexecuted limit orders for a particular contract, organised by price and time. On NSE, every futures and options contract has its own order book. Buy orders (bids) are ranked from highest to lowest price; sell orders (asks) from lowest to highest. The best bid and best ask sit at the top of their respective sides, and the gap between them is the bid-ask spread. Market depth is the visible snapshot of the top five levels on each side.
Why it matters
Understanding the order book helps you predict fill quality before you trade. A book with hundreds of lots queued at each of the first five price levels will absorb your order with minimal impact. A thin book — common in mid-cap stock options or far-OTM Nifty strikes — can gap several ticks just from a 10-lot market order. Every time you submit a market order on a thin book, the order book is the mechanism through which you pay that cost.
The order book also reveals information about intent. A sudden flood of large sell orders hitting the book (visible as a rising ask-side wall) can signal that a large participant is exiting or hedging a position. Conversely, a buy wall appearing just below the current price suggests strong support. Experienced traders watch these patterns to time their own entries and exits, though order book signals can be faked through "spoofing" — placing and quickly cancelling large orders to create a false impression of demand. SEBI prohibits spoofing, but it can still occur.
For options specifically, the order book interacts with the underlying's movement constantly. As the spot price of Nifty or Bank Nifty shifts, option market makers rapidly update their quotes in the order book, cancelling and replacing orders across dozens of strikes simultaneously. This is why the order book for options refreshes much faster than for equities — it is tied to the continuous repricing of the underlying's risk.
How it works
NSE uses price-time priority matching: within the same price level, the order that arrived earliest gets filled first. When you submit a limit buy at ₹150, your order joins the queue of all existing bids at ₹150. If a seller posts a sell order at ₹150, the oldest buy order at that price gets matched first. This "first come, first served" rule within a price level is why queue position matters for high-frequency participants competing to provide liquidity.
Example
Suppose you open the order book for a Nifty 24,000 CE expiring this Thursday. The book shows: Bid — 120 lots at ₹180, 85 lots at ₹179.95, 60 lots at ₹179.90. Ask — 95 lots at ₹180.50, 70 lots at ₹180.55, 50 lots at ₹180.60. You want to buy 100 lots. A market buy order would fill 95 lots at ₹180.50 and the remaining 5 lots at ₹180.55 — an average price of ₹180.503. A limit order at ₹180.25 (between bid and ask) sits in the book and waits; it may or may not get filled depending on whether a seller is willing to come down. Most retail traders in liquid Nifty strikes get fills within a tick or two of their limit price during normal market hours.
Trade where the book is deep
TradePulse shows open interest and volume across every strike so you can choose contracts with robust two-sided order flow.