Line a rimmed baking sheet with parchment paper or wax paper. Roughly chop or break the Oreo cookies into large pieces, reserving a few pieces for the top if desired.

Place the white candy coating in a microwave-safe bowl. Microwave in 20- to 30-second intervals, stirring thoroughly after each interval, until smooth and fluid.

Pour the melted white coating onto the lined baking sheet and spread it into an approximately 1/4-inch-thick layer. Press the chopped Oreo pieces into the soft coating before it begins to set.

Melt the red candy melts and blue candy melts separately in microwave-safe bowls, using 20- to 30-second intervals and stirring after each one. Drizzle both colors over the white bark in thin, decorative lines.

Immediately scatter the red, white, and blue sprinkles over the surface. Add the optional red and blue M&M's or other patriotic candy pieces while the coating is still wet so the decorations adhere.

Refrigerate the tray for 20 to 30 minutes, or until the bark is completely firm. Break it into irregular pieces by hand or cut it with a sharp knife for cleaner portions.