In a large bowl, combine oats and nuts. In a small saucepan, combine butter, brown sugar, espresso, water, and salt over medium. Cook, stirring occasionally, until butter melts, about 2 minutes. (Or, place ingredients in a medium microwave-safe bowl and microwave until butter melts, 1 to 2 minutes; stir until combined).
Pour butter mixture over oat mixture and toss until oats and nuts are completely coated. Transfer to a parchment-lined rimmed baking sheet and spread in an even layer. Bake until oats are lightly golden, 30 to 35 minutes, stirring every 10 minutes. Let cool completely on a sheet on a wire rack, 20 minutes, then stir in banana chips.
Store in airtight containers at room temperature, up to 3 weeks.