"Open Source" meaning the license is OSI approved (or at least meets the definition for "Open Source" by the OSI[1]) and source available is anything to which you can get the source to, but the license doesn't meet the above criteria.

[1]: https://opensource.org/osd

Accepting patches isn't a requirement, but it roughly means that you can make your own changes, publish those changes, and use the software for whatever you want. These don't automatically come with being allowed to view the source code.

The latter meaning the four freedoms or something equivalent (e.g. complying with the OSD and/or the the DFSG). They don't have to accept patches upstream but they do have to permit sharing your patches with other users one way or another.