Gartman's comments come amid tumbling oil prices, with Brent crude falling below $66 a barrel on Tuesday – its lowest level in five years. The price has fallen over 40 percent since June on the back of a global over-supply and lack of demand.
In November, the group of 12 major oil producers, OPEC, decided not to cut production at its meeting in November, further fueling sliding prices.
Read MoreOil falls as OPEC opts not to cut production
Some OPEC members, such as Iran, Venezuela and Ecuador, indicated that they were keen to cut production in order the stem the fall in prices. But OPEC's largest oil exporter, Saudi Arabia, refused to budge.
Gartman said the leadership position of Saudi Arabia – the main swing producer in the region – had "strengthened".
"(Now) they (OPEC) are doing exactly what the Saudis want to be done, as the Saudis continue to defend their market share so that they can put pressure on their enemies the Iranians, their other enemies of ancient times, the Russians and their new enemies, the frackers (U.S. shale oil producers)," he said.
Read MoreOPEC needs to 'wake up' to shale revolution
"But are the other members of OPEC happy about what's happening? Is Venezuela or Iran happy with what's going on? Of course not. I think we've seen the end of OPEC as a viable entity."
It's not the first time that Gartman has criticized the group of oil exporters. Following OPEC's November meeting, Gartman told CNBC: "The fact that OPEC, for all intents and purposes, is broken apart and is really now a broken cartel only serves to make certain that crude oil prices are still going to fall even further".
Global markets fell overnight as oil prices continued to decline in the Asian trading session. The decline in oil hit U.S. stocks hard on Monday, with the Dow Jones Industrial Index posting its biggest decline since October.
Read MoreDow's biggest dip since October as stocks hit by oil