Omnicom Posts Higher Quarterly Profit  NEW YORK (Reuters) - Omnicom Group &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=OMC.N target=/stocks/quickinfo/fullquote"&gt;OMC.N&lt;/A&gt; on Tuesday  reported a higher quarterly profit as the world's largest  advertising group added several high-profile accounts to its  roster and benefited from higher global ad spending.