The DJI OSMO is a nice 4K Video Camera + Gimbal system, but has the worst audio ever. In fact it's pretty much non-existent. But with a microphone attached to the 3.5mm input you can get much better results. I've played around with a few setups, but probably one of the best microphones to use with this tiny setup would be RODE's new VideoMicro On-Camera Hypercardioid Microphone.
Rode VideoMicro On-Camera Hypercardioid Microphone
In order to mount this with the OSMO they recommend the Straight Arm accessory along with the Universal Mount. This setup will let you use the SmartPhone Mount simultaneously (as seen below). Interestingly enough as pure convenience you can now purchase the Rode VideoMicro on the DJI Website for just $59 dollars.