Preheat oven to 350 degrees F. Line an 8 inch square pan with foil and grease generously with softened butter.  These are sticky!  If you have non-stick foil, you can use that instead and skip the extra butter.

In a large microwave-safe bowl or in a 3 quart saucepan, melt 1 tablespoon of butter.   Remove from heat and stir in the condensed milk, oats, and mixed nuts.  If you are adding any kind of chocolate chips and the mixture is still warm, let it cool slightly.  This would only be an issue if you melted the butter over the stove and there was still some residual heat.  Press down tightly and bake for 20 to 25 minutes.

Cool for about 20 minutes, then lift from pan and score while warm, then let cool completely.  You can throw them in the freezer if you want them to firm up quickly.