"I just bought a pair of navy oxford flats. Since most of my wardrobe is black, I have no idea what to wear them with! Any outfit suggestions?"
The oxford flat is such a classic style and so inherently casual that you should avoid trying to dress it up. Regarding your color challenge, I love pairing navy with black. In fact, I do it myself! I acknowledge that this is rule breaking, but what are rules for anyway? The key is making sure there's a discernible difference between the navy and black. So opt for a deeply saturated color, not one that flirts with navy, as in "navy-ish"; still, the color shouldn't be so dark that it reads as being almost black, which is midnight. You want harmony and balance, too. Use your visceral eye. It won't betray you!