The latest 13F season is here, when many money managers issue required reports on their holdings. It can be worthwhile to pay attention, as you might get an investment idea or two by seeing what some major investors have been buying and selling.

For example, consider Paulson & Co., founded in 1994 by investing giant John Paulson. Owned by its employees, the company has specialized in merger arbitrage, among other things, profiting when one company buys or merges with another (or merely announces plans to do so). It has grown into one of the largest hedge fund companies in the world.

Paulson & Co.'s latest 13F report shows that it has initiated or increased its positions in Dollar General (NYSE:DG), Oasis Petroleum (NYSE:OAS), and Vanda Pharmaceuticals (NASDAQ:VNDA).

Dollar General and other discount retailers saw their businesses boom when our economy went bust, during the recent recession. The company is still growing, but its growth rate has slowed -- due in part to larger rivals such as Wal-Mart introducing smaller stores and promoting price wars that have been shrinking Dollar General's profit margins. Reduced food stamp benefits have hurt the company, but including tobacco in its product mix has helped boost sales. With Dollar General's forward P/E ratio near 15 and well below its five-year average of 20, the stock seems appealing. But it carries significant debt, and the company will need to compete successfully against some deep-pocketed rivals. It pays no dividend, but it has been rewarding shareholders via share buybacks.

Oasis Petroleum has been building a big position in the promising Bakken shale region, and it will spend about $1.4 billion to explore and produce oil there. Its revenue has surged tenfold over the past few years, and its net margin was recently above 20%, but it has been burning through increasingly large sums of money as it invests in its future. Still, with its rapid growth rate and a forward P/E ratio near 14, its stock is compelling. It's not without risks, though, as it engages in the controversial practice of fracking. If fracking were to be prohibited or significantly restricted by regulations, Oasis would take a big hit. In the meantime, it has been lowering costs in part via vertical integration. It's an intriguing opportunity for those not averse to risk.

Vanda Pharmaceuticals doesn't look too attractive if you just glance at its financials. Revenue has been close to flat over the past few years, while it has been burning through cash and posting net losses. The company is focused on treating schizophrenia and sleep disorders. Its Hetlioz drug, approved by the FDA in January, treats non-24-hour sleep-wake disorder in blind people and has had its application for approval in Europe accepted. Approval there could double sales, and with no other drugs on the market tackling this disorder, Vanda has pricing power. Still, some have insufficient faith in management, which fumbled the launch of the company's schizophrenia drug Fanapt, and see the stock as overvalued at recent levels. A wait-and-see approach might be smart for those interested in Vanda.