To make the pineapple filling: Mix the crushed pineapple and juice, the sugar, corn flour, butter and water in a saucepan. Cook until thickened then cool.
Preheat oven to 200 degrees C.
Cream together 185g of butter and brown sugar thoroughly. Stir in flour, bicarbonate of soda and salt. Mix in rolled oats. Press and flatten half of the mixture over bottom of a 33 x 24 x 5cm greased slice tin. Spread with the cooled pineapple filling.
Top with remaining crumb mixture, patting lightly. Bake 25 to 30 minutes, or until lightly browned. While warm cut into slices and remove from tin.