Two people take opposite sides of the same SENSEX weekly option. One pays a few thousand rupees and is done. The other has to keep more than a lakh parked with the broker and watch it move every hour.
Same contract. Completely different obligation. Understanding why is the single most useful hour a new F&O trader can spend, and almost nobody spends it.
You walk up to a stall and pay ₹20 for a vada pav. That ₹20 is the most you can lose in this transaction. If the pav is terrible, you are out ₹20. Not ₹200, not ₹2,000. Twenty.
The stall owner is in a different business. He collects ₹20 from each customer, many times a day, and most days he goes home with a small profit. But he had to buy the pan, the gas cylinder, the potatoes and the bread before a single rupee came in. And if the gas price doubles overnight or the corporation shuts his lane for a week, his loss is not capped at ₹20. It is capped at whatever he has.
That is the option buyer and the option seller, exactly.
Say the SENSEX is near 77,300 and you buy one lot of the 77300 call at a premium of ₹150. SENSEX lot size is 20, so you pay ₹150 × 20 = ₹3,000, plus charges.
That ₹3,000 leaves your account and becomes the seller's money immediately. What you get in return is a right, not an obligation — the right to benefit if the index moves above your strike far enough to cover what you paid.
If the index goes nowhere, the option expires worthless on Thursday and you lose ₹3,000. If the index falls 800 points, you lose ₹3,000. If it crashes 2,000 points, you lose ₹3,000. The number does not get worse. It cannot.
The person who sold you that call received your ₹3,000. Their best possible outcome on this contract is that ₹3,000 — that is the ceiling, collected on day one.
Their obligation, though, is open-ended. If the index runs 1,500 points past the strike by Thursday, they owe the difference. That amount grows with every point the index moves.
The exchange is not willing to take that risk on faith. So it demands margin — a deposit blocked in the seller's account for as long as the position is open. On SENSEX weekly options this is typically well over a lakh of rupees per lot, and it is not a fixed number. It is calculated by the exchange's SPAN and exposure system, and it goes up when volatility goes up.
Margin is not a fee and it is not the broker being difficult. It is the clearing corporation making sure that when the seller is wrong, the money to settle is already sitting there.
Three consequences follow, and each one catches people out:
Buying caps your loss and hands you a clock. Time decay works against you every single day, and on a Thursday expiry it works against you every hour. Most weekly options bought out of the money expire at zero. The capped loss is real, and so is the fact that it is hit often.
Selling removes the clock problem — decay is on your side — and replaces it with a capital problem and a tail problem. You need serious money blocked to hold the position, and one gap-open against you can erase many weeks of collected premium.
Neither structure is superior. They are two different bargains with the same contract, and the exchange prices the difference in the only honest currency it has — margin.
This is educational material about how SENSEX weekly option contracts are structured. It is not a recommendation to buy or sell anything.