Check out <android/storage_manager.h>
- there are functions for mounting, unmounting, and getting the path of your obb file:
/**
* Attempts to mount an OBB file. This is an asynchronous operation.
*/
void AStorageManager_mountObb(AStorageManager* mgr, const char* filename, const char* key,
AStorageManager_obbCallbackFunc cb, void* data);
/**
* Attempts to unmount an OBB file. This is an asynchronous operation.
*/
void AStorageManager_unmountObb(AStorageManager* mgr, const char* filename, const int force,
AStorageManager_obbCallbackFunc cb, void* data);
/**
* Check whether an OBB is mounted.
*/
int AStorageManager_isObbMounted(AStorageManager* mgr, const char* filename);
/**
* Get the mounted path for an OBB.
*/
const char* AStorageManager_getMountedObbPath(AStorageManager* mgr, const char* filename);
Then you should be able to manipulate files as you normally would.