To calculate the number of pieces into which Oscar divided the 4/5 kilogram of clay, we need to divide the total weight of the clay by the weight of each piece. We can do this by first converting the weight of the clay to kilograms and then dividing it by the weight of each piece. The weight of the clay in kilograms is 4/5 * 1 kilogram = 0.8 kilograms. And the weight of each piece is 1/5 * 1 kilogram = 0.2 kilograms. So, to find the number of pieces into which Oscar divided the clay, we divide the total weight of the clay by the weight of each piece, giving us 0.8 kilograms / 0.2 kilograms/piece = <<0.8/0.2=4>>4 pieces. Thus, Oscar divided the 4/5 kilogram of clay into 4 pieces.