By Ahmad Ghaddar

LONDON (Reuters) -An OPEC technical panel invited a top financial market dealer to give a presentation this week which painted a bearish outlook for the oil market, according to materials from the presentation seen by Reuters.

"Market sentiment had been fragmented for much of this year, but the evidence is that there has been a recent shift in collective sentiment to bearish as we head towards the end of the calendar year," one of the presentation slides by Onyx Capital Group showed.

OPEC - the Organization of the Petroleum Exporting Countries - did not immediately reply to a request for comment.

London-based Onyx Capital Group is the world's biggest market maker by volume across oil swaps, trading more than 25 billion barrels equivalent a year. It says it owns a dataset that analyses market positions to predict how the market behaves.

Onyx CEO Greg Newman gave the presentation, according to a company tweet on X, formerly Twitter. Its content was not made public, however.

According to the materials seen by Reuters, it also showed that Brent futures had undergone two significant sell-offs since the start of the quarter, with the first, from Sept. 27 to Oct. 2, driven by weak U.S. gasoline markets and financial speculators.

Saudi Energy Minister Prince Abdelaziz bin Salman, OPEC's most influential minister, famously warned financial speculators against betting heavily in the oil market in 2020, challenging those looking to short the market to "make my day".

The second sell-off, which happened in November, moved the oil market to a collective neutral-to-bearish sentiment, with commercial participants like oil producers and airlines joining financial speculators in seeing a weak outlook.

"The second aggressive moves were correlated with a weakening forward outlook for underlying crude and refined products market," Onyx said in the presentation materials.

It added that the oil market forward curve signalled the market expects supply of heavy crude, which several OPEC members produce, will likely come back next year, and inventories will start to rebuild.

The Economic Commission Board (ECB), which OPEC describes as its "think-tank", meets twice a year ahead of the ordinary ministerial OPEC conference to review market conditions and economic developments.

While it has no decision-making capacity, the research it produces is important for ministers that will eventually decide on output policy.

Ministers from OPEC and allies led by Russia, a group known as OPEC+, are due to meet on Sunday to decide on output policy.

The group is likely to discuss output cuts to shore up falling oil prices, which are down more than 13% since the start of this quarter.

Goldman Sachs this week said it expected Saudi Arabia and Russia to extend their voluntary oil production cuts into the first quarter 2024.

It also sees a 35% possibility the group will deepen production cuts, potentially through another 0.5-1 million barrel per day reduction shared among the large producers, including Saudi Arabia, Russia, the UAE, Iraq, and Kuwait.

The ECB on Tuesday also heard presentations from banks JPMorgan and BNP Paribas, three sources told Reuters. It will meet for a second day on Wednesday.

(Additional reporting by Dmitry Zhdannikov; Editing by Jan Harvey)