Heat oven to 350°F. Line two baking sheets with parchment.
Whisk oats, flour, coconut, both sugars, and salt in a large bowl.
Melt butter with golden syrup in a small saucepan over low heat.
Stir baking soda into the boiling water in a cup to dissolve.
Pour the soda mixture into the melted butter. Stir; it will foam.
Add foamy butter mixture to dry ingredients. Mix until evenly moistened.
Scoop 1-tbsp portions onto sheets, spacing 2 inches. Gently flatten tops.
Bake 10–12 minutes until deep golden at edges. Cool 5 minutes, then rack.