Both are must have items if you are thinking about buying an Osmo. The Fleximic is $10, the 360 mic mount is $9.
I have not yet compared the Fleximic vs other microphones I have for the Osmo, I am still waiting for one more to arrive tomorrow, but I will have that up by this weekend. I will say, if you buy an Osmo, an external microphone is a must if you want to record sound, if you own one you already know the built-in microphone is a turd!
Purchase the Fleximic from DJI.com; FM-15 Flexmic $10
Purchase the mic mount from DJI.com; Quick Release 360 Mount $9