Iran has been locked out of the global oil market for years, and a new deal with the US could change that the moment both sides sign - with no phase-in or waiting period.
That kind of speed is unusual for sanctions deals, which usually unwind over months or even years.
A US official said any agreement with Iran would let Tehran start selling oil the second both sides sign, a sharp shift from how sanctions relief usually works in stages.
Iran still ships crude today - mostly to China through discounted, off-the-books channels - but a deal would open the front door for buyers who've been shut out since 2018, when the Trump administration pulled out of the original nuclear agreement and snapped sanctions back on.
Before sanctions snapped back, Iran was pumping nearly 4 million barrels a day and exporting roughly two-thirds of that, putting it in the same league as Iraq among major Middle East producers.
European refiners and Asian buyers outside China could line up without fear of US penalties, which means Iranian barrels would compete head-on with Saudi, Iraqi, and Russian supply.
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Iran sits on the world's third-largest oil reserves, and even a partial return to full export capacity could add over a million barrels a day to global supply.
That kind of jump tends to push prices one direction: down.
Lower crude usually means cheaper gas at the pump, but it also squeezes US oil producers who need higher prices to make new drilling worth it.
OPEC+ has been holding back production for over a year to keep prices propped up, and a wave of Iranian barrels would undercut that effort within days.
Brent crude is already trading near $80, with prices sliding to a three-month low as traders price in the looming deal.
US shale producers are especially exposed to that downside.
Many need crude above $65 a barrel to justify new wells, and a sharp drop could stall drilling plans across Texas and North Dakota.
No deal has been signed yet, and talks have stalled before - any agreement would also face pushback in Washington, where lawmakers on both sides have raised concerns about Iran's regional behavior.
But the structure of any deal matters as much as the timing.
An immediate-sale clause means there's no buffer between signing and supply - the market would reprice the same day.
Traders are already positioning for that scenario, with options markets tilting toward lower oil prices in coming months and some funds betting the deal gets done before year-end.
The next signal will come from the negotiating table, not the pump.
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