Preheat oven to 325° with racks in upper and lower thirds.
In a medium bowl, mix oats, sugar, and butter together with a wooden spoon. Stir in egg until well blended, then vanilla and orange juice. Add flour and salt and stir to mix. Batter will be somewhat thick.
Line 2 baking sheets with parchment paper or nonstick baking liners. Drop batter by half-teaspoonfuls onto sheets, at least 2 in. apart (cookies will spread), up to 12 per sheet. Bake until golden brown, switching positions of sheets halfway through, 8 to 10 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 to cooling racks and let cool completely.
Put chocolate chips in a microwave-safe bowl and heat in 20-second bursts until almost melted. Stir to melt completely.
Thinly spread chocolate on flat side of half of cookies. Sandwich with another cookie and let harden on racks, about 2 hours at room temperature.
Make ahead: Up to 1 week, airtight at room temperature.