Programmatically getting SharePoint Site Columns

Here is a simple code snippet of how to get all the available Site Columns under a given site.

public string GetSiteColumns(string site)
{
  string sRet = string.Empty;
  using (SPSite oSPSite = new SPSite(site))
  {
   using (SPWeb oSPWeb = oSPSite.OpenWeb())
        {
         foreach (SPField field in oSPWeb.Fields)
                {
                 if(null == field) return null;

                        //if(field.Group.ToLower() == "srini")
                        //{
                            sRet += field.Title.ToString() + ",";
                        //}
                 }
         }
  }
  return sRet;
}