NEW YORK (TheStreet) -- Overseas Shipholding (OSG), an operator of oil tankers, wasn't able to turn a profit in the first quarter, surprising most of Wall Street, which had expected rising rates for the largest ships in the company's fleet to buoy its results.

As the broader equities market sold off sharply Tuesday, with Europe's sovereign debt crisis taking an abrupt turn for the worse, shares of OSG, as the company is often called, plunged harder than other names in the oil tanker sector.

OSG's stock was changing hands in recent trades at $48.12, down $5.01, or 9.4%, on volume of nearly 1 million shares, eclipsing the daily average of about 770,000 shares.

The company, which hires out more of its fleet on the spot market compared to most of its peers, blamed the shortfall on weaker-than-expected rates among its smaller ships, especially the Aframax size vessels that take oil from West Africa to the U.S.

OSG also owns a considerable number of Very Large Crude Carriers, or VLCCs, among the biggest oil tankers in the world, which had enjoyed a sharp run-up in rates on the spot market in recent months. Strength in that portion of the business wasn't enough to help, apparently.

OSG posted a net loss of $9.4 million, or 34 cents a share. A year earlier, OSG earned $122 million, or $4.53 a share. Stripping out a series of items, OSG said, it lost 9 cents a share during the just-ended quarter. Revenue, meanwhile, came to $222 million, down 21% from a year ago.

The range of analysts' estimates was enormously wide, however, which points up the confusion that exists on Wall Street when it comes to handicapping tanker company earnings. That's because shippers rarely provide guidance on revenue or on operating expenses, especially if they have ships looking for work on the spot market, where rates can and do fluctuate wildly. Add in the tremendous operating leverage of the tanker industry in general, and any small discrepancy among analysts' opex projections become amplified in their bottom-line projections.

Per-share estimates for OSG ranged from a loss of 54 cents on the low end to a profit of 81 cents on the high end.

Doug Mavrinac, otherwise bullish on OSG, was at the low end, with that 54-cents loss as his target. He called the drop in OSG shares on Tuesday a "buying opportunity," saying the outlook for the second half of the year looks bright; he expects OPEC production to increase as the global economy continues to improve.

According to Mavrinac, in the company's conference call Tuesday morning executives said that a recent trip to the Far East suggested to them that there were no signs that demand was falling off, in that part of the world at least.

Still, OSG has a significant fleet of U.S. coastwise ships, which deliver oil from the Gulf of Mexico to American refineries, a market that had until recently performed poorly amid the recession.

On Monday, tanker stocks shot higher as the BP ( BP) oil spill threatened to disrupt offloading operations in the Gulf, which would tilt the vessel supply-demand balance and send shipping rates higher.

Elsewhere among tanker stocks Tuesday afternoon, those gains from Monday's session evaporated. Frontline ( FRO) was losing 5.5%, Nordic America ( NAT) was declining 2.5%, and Teekay ( TK) was giving up 2.8%.

Nordic, Teekay and Frontline are slated to report quarterly results later this month -- on May 10, and May 13, and May 28, respectively.

-- Written by Scott Eden in New York

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Scott Eden has covered business -- both large and small -- for more than a decade. Prior to joining TheStreet.com, he worked as a features reporter for Dealmaker and Trader Monthly magazines. Before that, he wrote for the Chicago Reader, that city's weekly paper. Early in his career, he was a staff reporter at the Dow Jones News Service. His reporting has appeared in The Wall Street Journal, Men's Journal, the St. Petersburg (Fla.) Times, and the Believer magazine, among other publications. He's also the author of Touchdown Jesus (Simon & Schuster, 2005), a nonfiction book about Notre Dame football fans and the business and politics of big-time college sports. He has degrees from Notre Dame and Washington University in St. Louis.

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