Pour the orange juice into a large (16 oz/47cl) glass.
Add ice to the glass, filling it to the brim.
Combine the hot water and matcha in a separate small bowl or glass. Whisk to combine, ensuring there are no lumps. You don't need a bamboo whisk. I use a silicone pastry brush but a small whisk of any sort (or a fork as a last resort) will do.
Pour the matcha over the ice. Serve.