What part stops you? The OGL 1.0a license itself is not OGC. It says what OGC is. OGL specifies that all of the rest(everything not explicitly excluded) of SRD 5 is open content as described in 1(d). The license terms describe that you are adding OGC as per 1(d) and anything not OGC is PI.
You could never use PI without permission by the 3PP creator. Original OGC is not under the control of WotC. And you can freely use everything else that is OGC from SRD 5.1 or its derivatives while SRD 5.1 is in CC. So again, what is stopping you from using a 5e 3PP product?
Nothing about CC-BY lets you use derivatives of CC-BY content, unless the author of that derivative gave you permission to.