Preheat oven to 180°Cj350°F/Gas Mark 4. Prepare two Swiss roll tins by greasing or lining with baking parchment.
Sieve the flour and cocoa into a large mixing bowl and stir in the oatmeal and sugar.
Melt the margarine in a saucepan and pour over the dry ingredients, mixing well.
Pour into two prepared baking tins and smooth out with a palette knife.
Bake in preheated oven for about 25 minutes before transferring to a wire rack to cool.
When the bar bases are cool, make up the icing one batch at a time.
Mix together half the sugar, orange juice and cocoa powder and smooth over the first bar base, marking out lines on the icing for dividing into squares or rectangles of required size.
Set aside to harden and repeat with the re maining icing ingredients for the second base.
Once the icing has hardened, cut into bars along the markings.
Store in an airtight container, where they will keep well for about a week.