In a large bowl, mix the flour with all the other ingredients up to and including the salt.
Pour the orange juice, oat milk and oil into the dry ingredients, mix to form a homogeneous batter. Transfer to the prepared tin.
Approx. 30 mins. in the centre of an oven preheated to 175 °C (convection). Remove from the oven, allow to cool slightly, remove the tin frame, leave to cool on a cooling rack.
Beat together the margarine and icing sugar using the whisk on a hand mixer until the mixture becomes lighter in colour. Add the cream cheese, reserved orange juice and food colouring, mix briefly. Spread the frosting over the cake, smooth down.