arcpy - Using gdal functions from OSGeo4W out of their shell?

I would like to use some gdal functions from OSGeo4W (gdal2xyz, and gdal_merge) for my work. I can use them in the OsGeo4W shell provided after installing the package. But I want to use them in Pythonout of the OSGeo4W shell. I have Python2.7, gdal, gdal-python binding installed (all for 64 bit). However, I don't have a clue on how to use those gdal functions out of their shell.

The reason I want to do this is that I have a module created from ArcGIS ModelBuilder which I exported as a Python script. With this script, I want to use gdal2xyz, gdal_merge, etc. to continue data process.