Preheat oven to 325°F with racks placed in upper and lower thirds of the oven.  Line 2 baking sheets with parchment paper or nonstick baking liners.

In a medium bowl, mix oats, sugar & butter together with a wooden spoon or spatula.  Stir in egg until well blended, then vanilla & orange juice.  Add flour & salt and stir to mix.  The batter will be somewhat thick and lumpy.

Drop batter by half-teaspoonfuls onto prepared baking sheets, at least 2 inches apart (cookies need room to spread), up to 12 per sheet. Bake until golden brown, switching positions of sheets halfway through, 8 minutes total.

Cool cookies on baking sheets until just holding their shape, about 2 minutes (don't wait until they're completely cool, as they will be harder to remove). With a thin metal spatula, gently transfer them to cooling racks and let cool completely.

Place chocolate chips in a microwave-safe bowl and heat in 20-second bursts until almost melted.  Stir to melt completely.  Allow the melted chocolate to rest for several minutes before spreading on cookies.

If some of the cookie wafers took on a unique shape as they spread, try to match them up with similar wafers. Thinly spread chocolate on flat side of half of the cookies. Sandwich with another cookie and let harden on racks, about 2 hours at room temperature.

Can be made up to 1 week ahead, store in airtight container at room temperature.