C:\>gdal_calc
Traceback (most recent call last):
  File "C:\OSGeo4W\\bin\gdal_calc.py", line 47, in <module>
    import gdal
  File "C:\OSGeo4W\apps\Python27\lib\site-packages\gdal.py", line 2, in <module>

    from osgeo.gdal import deprecation_warn
  File "C:\OSGeo4W\apps\Python27\lib\site-packages\osgeo\__init__.py", line 21,
in <module>
    _gdal = swig_import_helper()
  File "C:\OSGeo4W\apps\Python27\lib\site-packages\osgeo\__init__.py", line 17,
in swig_import_helper
    _mod = imp.load_module('_gdal', fp, pathname, description)
ImportError: DLL load failed: The specified procedure could not be found.