Just as investors have accepted the likelihood of a December rate hike, another big uncertainty resurfaced: the price of oil.

Worries over how low oil prices could fall returned with a force on Monday, heightening concerns that the steep selloff is only the beginning of a renewed downward push for equity markets.

Crude oil closed at its lowest level since February 2009 after a recent meeting of the Organization of Petroleum Exporting Countries failed to produce significant guarantees to reduce output. That's a less than encouraging message given that global production is at near-record highs even as demand dissipates. 

West Texas Intermediate crude fell 5.8% to $37.65 a barrel prompting declines across the board: the S&P 500 fell 0.7%, the Dow Jones Industrial Average declined 0.65%, and the Nasdaq slid 0.79%.

"Oil prices are on the decline in the aftermath of Friday's OPEC meeting which failed to result in agreement on output targets," Lindsey Piegza, chief economist at Stifel, wrote in a note. "Already, energy analysts are discussing the possibility of $20 a barrel which could pose serious headwinds to next year's growth outlook."

Take Goldman Sachs analyst Michele Della Vigna, for example, who told BBC Radio 4's Today show on Monday that a $20-a-barrel level could be reached due to a shortage of storage facilities. However, he assured that a dip that low would be temporary before markets stabilized. 

Warren Gilman, chairman and CEO of CEF Holdings, also supported the possibility of a dip as low as $20 in 2016. "There is no question that prices are going to continue to fall," he told CNBC on Monday. "Will they break the $30s to the $20s? That's quite a possibility in 2016."

The factors that caused such wild swings in oil prices this year will likely continue into the next, according to Brian Milne, energy editor at Schneider Electric. 

"Volatility best describes the global and domestic oil markets in 2015," Milne wrote in a note. "As 2016 comes into focus, the themes of 2015 remain major factors that will direct crude oil prices. 

Those driving forces include continued weakening in global demand which will squeeze prices, while a glut in inventories will "stave off potential price spikes caused by geopolitical tension and supply disruptions. Milne predicted that an expected early-2016 price surge and stabilization will be pushed out towards the end of the year. 

The steep selloff in crude on Monday pushed the energy sector deep in the red. Major oilers including Exxon Mobil (XOM - Get Report) , Chevron (CVX - Get Report) , PetroChina (PTR - Get Report) , Royal Dutch Shell (RDS.A - Get Report) and BP (BP - Get Report) were more than 2% lower, while the Energy Select Sector SPDR ETF (XLE - Get Report) slumped 4.3%.

Gun stocks pushed higher on Monday after a speech from President Barack Obama on Sunday evening called for tighter regulation of firearms. Smith & Wesson (SWHC) and Sturm Ruger (RGR - Get Report) jumped around 6% each.

Office Depot (ODP - Get Report) shares tumbled more than 10% after the U.S. Federal Trade Commission filed a complaint against Staples (SPLS) for its proposed takeover. Regulators said the $6.3 billion acquisition would create the largest U.S. retailer of office supplies with little competition.

Chipotle (CMG - Get Report)  fell after the burrito chain warned that comparable sales would fall 8% to 11% in the fourth quarter as a result of an E. coli outbreak. Cases have recently been reported in three more states -- Illinois, Maryland and Pennsylvania. The outbreak has spread to nine states and affected 52 people.

Keurig Green Mountain (GMCR) shares surged 71% after the company agreed to be bought by JAB Holding in a deal worth $13.9 billion. Keurig shareholders will receive $92 in cash for each share they hold. The stock closed Friday at $51.70.