Fortress to Buy Stelmar Shipping  NEW YORK (Reuters) - Private equity firm Fortress  Investment Group LLC agreed to buy Stelmar Shipping Ltd. for  \$677 million only a few months after the oil tanker operator  fended off a hostile bid from its rival OMI Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=OMM.N target=/stocks/quickinfo/fullquote"&gt;OMM.N&lt;/A&gt;,  Stelmar said on Monday.