I would say that most people think of Android as the complete operating system experience that comes preinstalled on most Android phones, and that experience depends a lot on proprietary parts, especially Google Mobile Services that are apparently deeply integrated with the OS (could it be considered part of it at this point?). Plus, these proprietary parts are not fully nor easily replaceable as you almost certainly have to wipe the whole operating system to do so.
So, in my opinion, calling Android an Open Source OS is kind of misleading, but it's a matter of how you interpret what Android is. I would rather refer to Android as the final product (which relies on proprietary software), and to AOSP as the open source project which Android is built on.
Plus, I think that AOSP and Android are considered two different entities and I doubt that AOSP is under the trademark of Android.
To me Android is like freedesktop or GNU. It's a platform that apps target. Some of those apps may also be open source, but some of those may be closed sourced. There are a ton of different distributions for these operating systems and each one gives it it's own twist.
So, in my opinion, calling Android an Open Source OS is kind of misleading, but it's a matter of how you interpret what Android is. I would rather refer to Android as the final product (which relies on proprietary software), and to AOSP as the open source project which Android is built on.
Plus, I think that AOSP and Android are considered two different entities and I doubt that AOSP is under the trademark of Android.