At least i managed to start Pycharm with an OSGEO4W Environment. My PyCharm is 2017.3, 64bit-Edition. My OSGEO4W is 32bit and uses QGIS3/Python3. I think it will be easy to change it for QGIS2/Python2.7 or anything with 64bit.I do start PyCharm with the batch-file in PyCharms bin directory, called by a custom Script to set the needed environment variables, allowing to make QGIS-Plugins with Qt and QGIS-Standalone-Applications. When PyCharm opens I create a new Project and set the Interpreter to the QGIS python in %OSGEO4W_ROOT%\apps\Python36.