Blend the creamy base: Add the banana, maple syrup, oat milk, and protein powder to a blender. Blend until smooth and creamy, making sure the protein powder is fully incorporated.
Add the berries: Add the blueberries, raspberries, and strawberries to the blender. Blend again until the mixture is completely smooth and the berries are fully pureed. If you're using a high-powered blender, you can simply add all the ingredients at once and blend until smooth.
Fill the popsicle molds: Carefully pour the smoothie mixture into your popsicle molds, filling each cavity almost to the top. Gently tap the mold on the countertop a few times to release any large air bubbles and help the mixture settle evenly.
Insert the sticks: Place the lid on the popsicle mold if your mold comes with one. Insert a popsicle stick into the center of each mold, leaving about 1½ inches exposed so they're easy to hold once frozen.
Freeze until firm: Transfer the molds to the freezer and freeze for at least 8 hours, or overnight, until completely solid. If you're using frozen fruit, the popsicles may freeze a little faster.
Unmold and serve: When you're ready to enjoy your smoothie popsicles, fill a shallow pan with hot water. Dip the bottom of the popsicle mold into the hot water for about 10 seconds to help loosen the popsicles. Gently pull them out and serve immediately.