New York — Oil investors must be getting dizzy.

In the two months since Opec began talking about capping production, speculators’ sentiment has swung wildly, with government and exchange data showing the four biggest weekly position changes ever for the two global benchmark crudes. The latest shift is to optimism, with money managers the most bullish on West Texas Intermediate oil in two years.

“Since the summer we’ve had big moves in net length,” said Mike Wittner, head of oil-market research at Societe Generale SA in New York. “It usually has trended up or down over a couple of months. Now this is happening in a matter of weeks. We’re seeing huge shifts.”

Money managers reduced bets on lower WTI prices by more than half in the past three weeks as Opec agreed to its first deal to cut output in eight years. That drove net length to the highest since July 2014 in the week ended October 11, Commodity Futures Trading Commission data show.

The Organisation of Petroleum Exporting Countries (Opec) agreed on September 28 in Algiers to trim output to a range of 32.5 million to 33 million barrels a day, which is due to be finalised at the Vienna summit next month. Opec took a step toward coordinated supply curbs with Russia last week and will meet for a “technical exchange” to set a road map for output levels later this month.

The swings in sentiment have tracked the rocky road to $50 (Dh184) a barrel oil. Speculators’ combined WTI and Brent crude net position rose or fell more than 100,000 contracts four times in the past two months, the only moves of that size in CFTC and ICE Futures Europe data going back to 2011.

Prices began to rise after OPEC’s president said August 8 that the group would hold informal talks in Algiers and Saudi Arabia signalled August 11 it was prepared to discuss taking action to stabilise markets. Futures gave up most of those gains amid doubts that Saudi Arabia and Iran would reach a deal, before the agreement in Algiers sparked the latest rally.

“The change in tone from the Saudis is important,” said Kurt Billick, the founder and chief investment officer of Bocage Capital LLC in San Francisco, which manages about $432 million in commodities equities and futures. “Getting to a yes in Vienna is challenging. That they are willing to talk about a deal is a big change.”

Money managers’ short position in West Texas Intermediate crude, or bets on falling prices, shrank by 28 per cent to 71,407 futures and options. Longs rose 1.8 per cent to the highest since June 2014. The resulting net-long position increased 13 per cent.

WTI increased 4.3 per cent to $50.79 a barrel in the report week. Prices on Monday were down 0.3 per cent at $50.19 a barrel as of 12:17pm in Singapore.

Other markets

In other markets, net-bullish bets on gasoline rose 19 per cent to 36,650 contracts, the highest since March 2015, as futures slipped 1.1 per cent in the report week. Wagers on higher ultra low sulphur diesel prices climbed 46 per cent to 9,074. Futures rose 2.1 per cent.

The scale of the internal differences Opec must resolve before securing a deal to cut supply was revealed October 12 as the group’s latest output estimates showed a half-million-barrel difference of opinion over how much two key members are pumping.

“The bottom line is that they’ve made an agreement,” Wittner said. “If you are going short you are betting against the Saudis, which isn’t a good thing historically.”