On a parchment-lined baking sheet, spread banana slices in a single layer.
In a small heatproof bowl, microwave peanut butter in 10-second increments, stirring between each, until runny, 20 to 30 seconds total. Pour peanut butter over bananas, spreading with a spatula to cover. Transfer sheet to freezer and freeze until peanut butter sets, 6 to 8 minutes.
In another small heatproof bowl, microwave chips and oil in 15-second increments, stirring between each, until melted and smooth, about 1 minute and 15 seconds total. Pour chocolate over peanut butter and spread with spatula to cover peanut butter. Continue to freeze until chocolate is set, about 5 minutes.
Sprinkle salt over chocolate (if using). Continue to freeze until solid, at least 1 hour or up to overnight.
Break into pieces to serve.
Make Ahead: Bark can be made 2 months ahead. Wrap pieces in plastic wrap, store in an airtight container, and freeze.
