What LTC said. Oat bran is the ground up outer coating of an oat groat. It has the same nutrional profile as oats themselves, ie, some fiber, a smattering of protein and lots and lots of carbs. Oat fiber is derived from the hull of the groat and it is composed entirely of insoluble cellulose-based fiber. That’s why it’s carbless. When ground into flour, it makes a useful addition to low carb bread mixes, can be dissolved in water for a disgusting but effective fiber supplement, and when mixed in with a bunch of seeds, nuts etc. for a porridge, the yucky sludginess that makes it so terrible to drink dissolved in water works like a charm to add soft richness and a smooth base for the varying textures of the other ingredients.
It’s easy enough to find the US these days. I’m not sure where you’re located, Squidge, but I bet you can find an online supplier that can ship you oat fiber.
The mixed seeds/nuts/etc. could definitely be a muesli kinda thing. I make a trail mix, which is not dissimilar to muesli, from tamari almonds, pecans, flaxseeds, hemp hearts, sunflower seeds, spiced pumpkin seeds (pepitas) coconut flakes and pure unsweetened cacao nibs. High in healthy fats, protein and fiber, rock buttom low in sugar. Dried fruits you buy commercially are usually sweetened, and even in their natural state are so high in sugar they’d ruin the vibe, but coconut and, if you can find them, dried raspberries or bluberries with zero added sugar can be added in modest amounts without damning your carb counts.