Line an 8-inch/20-centimeter square pan with parchment paper.
In a medium bowl, stir together the peanut butter, maple syrup, oats, and nuts until evenly combined. Press the mixture firmly into the prepared pan.
Place the chocolate and coconut oil in a microwave-safe bowl and heat in 20-second intervals, stirring between each, until smooth. Alternatively, melt them together over a double boiler.
Pour the melted chocolate over the peanut butter mixture and spread into an even layer.
Freeze until the chocolate is firm, about 1 hour.
Cut into bars and serve. Store in an airtight container in the refrigerator for up to 1 week or in the freezer for up to 2 months.