Near 16:30 GMT, US benchmark West Texas Intermediate for delivery in February dropped to $49.95 a barrel, but then rose to $50.20 a barrel.
The February contract for Brent crude also touched a new five-and-a-half-year low, dropping to $53.08 per barrel.
The breach of the $50 mark came on a rocky day in global financial markets, with the Dow ending down more than 200 points and European stocks losing more than two percent.
"People are thinking about promises from OPEC, mostly Saudi Arabia, that they’ll continue to produce at very high levels," Bart Melek, head of commodity strategy at TD Securities, told AFP. "On the demand side of the equation, what we’re getting is basically a lack of demand growth… as Europe is potentially in crisis."
A weak euro may also have driven oil prices to new low as it reduces the purchasing power of euro holders for dollar-denominated oil.
Earlier on Monday, the euro hit a nine-year low against the US dollar on the Greek turmoil and speculation the European Central Bank is preparing for large-scale bond purchases.