This is probably because you modified (or overwrote) some .pk3 in your OA installation directory.
Can you check you have default .pk3 ?
(You may consider updating to 0.8.0
)
I'm not sure, but other (and similar) reason could be, you've got an additionnal .pk3 in your home directory which overwrites files present on default .pk3, you should then remove this pk3.