Preheat the oven to 180C. Grease and line a 20cm square tin with baking parchment.
Put the peanut butter, sugar, honey and oil in a saucepan and heat over a low-medium heat for a few minutes until the mixture is combined and the sugar has dissolved. Remove from the heat and stir in the raisins and oats.
Transfer the mixture to the tin and use the back of a spatula to press down and push the flapjack mixture into all four corners.
Bake in the hot oven for 20-25 minutes until golden brown. Remove from the oven and use a sharp knife to cut into 12 pieces while still warm. Allow to cool in the tin for 10 minutes before carefully transferring to a cooling rack to cool completely.