Eh, OSD 3 just says that derived works must be possible, it doesn't say that you must be able to incorporate third party source code into the derived work. Meanwhile the FSF's definition explicitly calls out this freedom as an essential component of Freedom 1:
> One important way to modify a program is by merging in available free subroutines and modules. If the program's license says that you cannot merge in a suitably licensed existing module—for instance, if it requires you to be the copyright holder of any code you add—then the license is too restrictive to qualify as free.
