OPEC's latest warning that crude could stay below $100 a barrel for the next decade wasn't enough to keep Dennis Gartman on the sidelines in the oil market. To the contrary, the founder of "The Gartman Letter" said that he's now bullish on crude oil for the first time "in a very, very long while."

The Organization of the Petroleum Exporting Countries said in a draft of its latest strategy report that it doesn't expect crude to trade consistently above $100 per barrel in the next decade, according to the Wall Street Journal. The draft reportedly forecasted crude trading at around $76 per barrel in 2025, and also modeled scenarios where crude would trade below $40 per barrel in 2025.

Read MoreOPEC: Oil prices to stay below $100 for next decade

OPEC released a statement Tuesday saying it questioned the "factual accuracy" of the Wall Street Journal's report.

The pessimistic tone, if true, would be a big reversal from OPEC's forecasts over the last three years. In its 2012 World Oil Outlook report, OPEC said the nominal price of oil, as measured by the OPEC Reference Basket, would rise to $120 per barrel by 2025.

The cartel's 2013 report saw oil rising to $110 per barrel by 2020, and increasing to $160 per barrel by 2035. In 2014, OPEC said oil would reach $124 per barrel by 2025.

Despite OPEC's reportedly cautious turn, Gartman said Monday that now is the time to buy crude, at least for a short-term trade.

According to Gartman, OPEC's predictions should be taken "with a great grain of salt," and the real measure to watch is crude's term structure, or the difference between prices at future dates.

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"I pay very particular attention to the term structure, because I think that's where informed money leaves its footprints in the sand," he said on CNBC's "Fast Money."

When crude prices broke early last week, he added, the front month futures gained ground relative to the back months.

"In a break in crude oil, we should have seen the carrying charge widen, and it didn't," he said. That, according to Gartman, is a bullish indicator.

Gartman also said an overestimation of U.S. production could help fuel a rally. "If we have actually overestimated the amount of supply from the United States by maybe 1 million barrels per day…maybe it's time for crude oil to have a small little rally."

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