Christopher,

You need to use the CultureInfo class found in the System.Globalization namespace. This class will allow you to interrogate the current OS's codepage (OEM, Ansi, DOS, Mac) through the TextInfo class.

using System;
using System.Globalization;

public class Test
{
   public static void Main()
   {
      // Show them all.
      foreach (CultureInfo cInfo in CultureInfo.GetCultures(CultureTypes.AllCultures))
      {
         Console.WriteLine(cInfo.TextInfo.OEMCodePage);
      }

      // Show the culture associated with the OS.
      Console.WriteLine(CultureInfo.InstalledUICulture);

      // Show the culture associated with the current thread.
      Console.WriteLine(CultureInfo.CurrentCulture);
      Console.WriteLine(CultureInfo.CurrentCulture.TextInfo.OEMCodePage);
   }
}

Regards
Neil