Oaki wants to decorate the secret room that he made in the Deku Tree's mouth.
Quest Location
You can start this quest after you complete Trail of the Master Sword. Then go to the Spore Store in the Deku Tree, and use Ascend to reach Oaki's secret room. Talk to Oaki to start the quest.
How to Complete the Quest
Obtain four Blue Nightshades and two Giant Brightbloom Seeds, then talk to Oaki to complete the quest.
Quest Reward
Oaki gives you the Korok Fabric.