Very interesting question.
Kong Sang Koon definitely fits well with the Pyung ahn forms and I can see pieces of other forms as well. But this question makes me think of an analogy with the Chil Sung forms. If you learn the first 3 Chil Sung forms, there are pieces of the Pyung ahns all through out them, but still in a slightly different presentation. So I can see the argument that you may not need to learn the Pyung ahns if you know Kong Sang Koon.
As for Oh Ship Sa Bo, I can see pieces of the Naihanchis and well as some other hyung. Personally, I would say that it is valuable to learn both KSK and OSSB. Especially since I know how you think of forms. If you can dig into both of them and get the meanings out of the movements and analyze them, I could see you getting completely different things from the two forms.
Whereas, I can see you getting the same basic applications and ideas from KSK and the Pyung Ahns, as well as from OSSB and its derivative forms.
I am also a believer that every form has purpose in its presentation, order, and practice. There is also something to be learned just from the action of learning a form.
The part about OSSB fitting with a different grouping of hyung...well, that may be true as many portions don't seem to fit well with other parts of the TSD curriculum. But that is true of other forms too...when I first learned Yuk Ro Cho Dan, I was blown away by how much different it was, but it is really just in the difference of the Japanese vs Chinese influence.