Place oatmeal cookies in a ziplock bag and use rolling pin to crush the cookies until crumbled. Finely grind up the walnuts. Melt the butter. Mix together the cookie crumbs, walnuts, and butter until combined. Place parchment paper on bottom of a springform pan (26 cm diameter). Flatten crumb mixture onto the bottom of pan and let it cool in the refrigerator for 1-2 hours.