Heat the water just until small bubbles form on the bottom of the pot, well below a rolling boil. A thermometer helps here since oolong scorches easily at full boil.
Add the loose-leaf oolong and steep for exactly 4 minutes. Pull the leaves the moment the timer goes off to avoid bitterness.
Strain out the leaves and let the tea cool to room temperature before refrigerating.
Make the Peach Syrup
Combine the peach chunks, sugar or honey, and water in a small saucepan. Simmer over medium heat for 5-7 minutes, mashing the fruit as it softens.
Blend the softened peach mixture until smooth, then strain through a fine mesh strainer into a jar to remove the pulp and skins.
Assemble the Tea
Stir the cooled peach syrup into the chilled oolong tea, tasting and adjusting sweetness as you go.
Fill glasses with ice, pour the tea over the top, and garnish with a fresh peach slice.
Notes
For a stronger cold-brew version, steep the oolong overnight in the fridge instead of hot-steeping. Freeze extra brewed tea into ice cubes so the glass does not dilute as it melts. Store leftover tea and syrup separately for up to 3 days.