So after installing OSGeo4W, another Python has been installed on my system. My previous Python was 2.7.10 and this new one (in OSGeo4W folder) was 2.7.5. So, I completely erased OSGeo4W package following this postthis post and the new installed Python was removed as a result. Then, I followed instructions on this page exactly and managed to install osgeo package. It doesn't come with qgis and other softwares but I didn't need them anyways.
replaced http://gis.stackexchange.com/ with https://gis.stackexchange.com/