Android as a useable system is not open at all. If you attempt to install say a native Debian system onto a phone it will fail because of unsupported hardware (I don't consider useable something with no audio or networking, touch screen etc), so you must rely on precompiled binaries ripped by some Android ROM. At the same time, if you strip Android of all closed source code, the resulting image could probably be installed only on a virtual machine. So though technically we can agree on Android being mostly OSS, that "mostly" part alone would be unsatisfactory for about all users. That makes Android a closed system to me.