Place 1/2 cup (110g) sugar in a shallow bowl. Dip both sides of orange slices in sugar. Grease a large non-stick frypan and place over medium heat. Add orange slices and cook, turning once, for 2-3 minutes each side or until dark and caramelised. Transfer to a tray lined with baking paper and cool completely.
Place orange juice and remaining 85g caster sugar in a small saucepan over medium-low heat and stir to dissolve sugar. Bring to a simmer and cook for 5 minutes or until thickened.
Fill serving glasses with ice cubes. Combine gin, vermouth and Campari in a large jug. Divide among glasses, top with warm syrup and caramelised orange, and serve immediately.