Toast and chop pistachios (if not already toasted): Heat a small dry skillet over medium heat, add the pistachios and stir constantly for 2–3 minutes until fragrant and a few bits turn a touch darker. Toasting releases the pistachios' oils and amplifies their buttery flavor — chop roughly once cooled so you keep some larger crunchy pieces.

Segment the oranges: Using a sharp paring knife, cut a small slice off the top and bottom of each orange so it sits flat. Working over a bowl to catch juices, cut away the peel and pith, then slice between membranes to free the segments (supremes). Reserve any juices in the bowl — those juices will be folded into the oat mixture for added citrus brightness and natural sweetness.

Combine wet ingredients to flavor the base: In a large mixing bowl whisk together the Greek yogurt, milk, maple syrup, vanilla extract, cinnamon (if using) and the pinch of salt. Whisking mellows the yogurt’s tang and distributes the maple and vanilla evenly so every spoonful is balanced.

Add oats and citrus juice: Stir the gluten-free rolled oats into the yogurt-milk mixture, then pour in the reserved orange juices. The oats absorb liquid overnight and soften without cooking; the acid in the orange juice brightens the dairy and helps carry the maple and vanilla flavors.

Taste and adjust sweetness/seasoning: At this stage taste the base — the oats will mellow a bit after chilling, so you want it slightly brighter and sweeter than your ideal cold serving. Add up to an extra teaspoon of maple or a pinch more salt if needed. This step prevents bland overnight oats after refrigeration.

Portion into jars: Divide the oat mixture equally among 4 serving jars or airtight containers (about 1 1/4 cups per jar). Layer with orange segments — either fold half the segments into the oats (for integrated citrus pockets) and reserve the rest to top each jar, or keep all segments as a fresh topping. Layering creates contrast between creaminess and bright citrus bursts.

Chill overnight: Seal the jars and refrigerate for at least 6–8 hours (overnight is best). Chilling allows the oats to hydrate fully and the flavors to marry; the texture shifts from raw to creamy, similar to a thick pudding.

Finish before serving: Remove jars from the fridge 10–15 minutes before eating to take the chill edge off (this improves flavor perception). Stir briefly to loosen the mixture if it's very thick. Taste and add a splash of milk if you prefer a looser consistency.

Assemble toppings: Spoon any reserved orange segments on top, sprinkle the toasted chopped pistachios and — if desired — a tiny drizzle of extra maple syrup or a pinch of flaky sea salt. The warm-toasted nuts against the cool creamy oats and bright citrus make for an irresistible texture and flavor contrast.

Serve and store: Serve chilled. Leftover jars keep well refrigerated for up to 4 days — add fresh orange just before eating for best texture. If using almond milk, the shelf-life is similar but always smell and check before eating.