NEW YORK (TheStreet) -- Stocks were mixed Monday in a topsy-turvy trading session in which investors were torn between stabilizing oil prices and fears of political drama in Greece. 

The clear losers on stock markets Monday were Russian ADRs which slid after S&P downgraded the nation's sovereign credit rating to "junk" status, below investment grade. Mobile TeleSystems(MBT) - Get Report , VimpelCom (VIP) , Yandex(YNDX) - Get Report and Qiwi(QIWI) - Get Report plummeted, though were already trading lower on worsening violence in Ukraine and the threat of further sanctions against Russia. The Direxion Daily Russia ETF (RUSL) - Get Report tumbled more than 21%.

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The S&P 500 edged 0.16% higher, the Dow Jones Industrial Average fell 0.02%, and the Nasdaq added 0.2%. 

Crude oil prices stabilized from an earlier selloff following reassurance from the secretary-general of OPEC that oil prices would bottom out at current levels. "Now the prices are around $45-$55 and I think maybe they reached the bottom and will see some rebound very soon," Abdullah el-Badri told Reuters. El-Badri also said without new investment crude oil prices could recover to $200-a-barrel levels as the supply-demand dynamic restabilizes over the next few years.

West Texas Intermediate crude oil was down 0.3% to $45.44 a barrel.

Prices earlier fell more than 1% as Saudi Arabia's successor reiterated the country's commitment to previous oil production policies. The biggest OPEC producer has been under pressure to limit output in the face of global oversupply. 

Energy producers Exxon Mobil(XOM) - Get Report , Chevron(CVX) - Get Report , Royal Dutch Shell (RDS.A)  and BP(BP) - Get Report were all higher. The Energy Select Sector SPDR ETF (XLE) - Get Report climbed 1.4%.

Athens' ATG stock index was down 3.2% after anti-austerity party Syriza won the general election in Greece, igniting fears the new government could compromise the terms of Greece's bailout package, possibly leading to Greece's exit from the eurozone. Since 2010, the European Union and International Monetary Fund have loaned Greece 240 billion euros ($277.8 billion). 

"Markets appear to be uncertain about how to react to the Greek Syriza party's near-majority victory over the weekend," said TD Securities' Gennadiy Goldberg. "The risk-off sentiment ... appears to have eased as Syriza leadership insists that it wishes to keep Greece in the eurozone."

A flurry of dealmaking made headlines Monday. Paper packaging company MeadWestvaco (MWV) jumped 13.3% following an announcement it was merging with competitor RockTenn (RKT) . The merger, worth $16 billion, would see MeadWestvaco hold 50.1% of the combined company.

Energy Transfer Partners (ETP) agreed to purchase affiliate Regency Energy Partners (RGP) for around $18 billion. The combined company would have operations in the majority of oil-producing regions in the U.S. Energy Transfer was 6.4% lower and Regency Energy jumped 6%. 

PartnerRe (PRE) and Axis Capital Holdings(AXS) - Get Report agreed to merge in a transaction that creates a giant reinsurer valued at $11 billion. Axis climbed 7%.

Universal Display(OLED) - Get Report spiked 6.4% after signing a patent licensing deal with LG Display(LPL) - Get Report through to 2022. As part of the deal, Universal Display would receive license fees and royalties.

D.R. Horton(DHI) - Get Report climbed 5.8% as quarterly profit of 39 cents a share beat expectations and revenue jumped 38% year over year. Fellow homebuilders Toll Brothers(TOL) - Get Report , KB Home(KBH) - Get Report and PulteGroup(PHM) - Get Report were all higher.

Mattel(MAT) - Get Report tumbled more than 4% after announcing the resignation of CEO Bryan Stockton. The toymaker also slashed its fourth-quarter profit guidance to 52 cents a share, well below consensus of 91 cents.

Seagate Technology(STX) - Get Report was nearly 8% lower following weaker-than-expected guidance. The hardware maker guided for third-quarter sales of $3.45 billion, less than estimates of $3.59 billion.

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--Written by Keris Alison Lahiff in New York.