In a medium-sized saucepan set over medium heat, melt and stir together the butter, sugar, and syrup, cooking until the sugar has dissolved. Stir in oats, flour, salt, and graham cracker crumbs.
Press slightly more than half of the mixture into a lightly greased 9 x 9 inch pan. Let cool completely. Preheat the oven to 350F. Sprinkle the chocolate chips evenly over the top, then the marshmallows. Top with remaining crust mixture. Bake the bars for 15 to 20 minutes. Remove from the oven and let them rest for 20 minutes, then cut into squares or bars while still slightly warm.