oak
槲 refers specifically to a type of oak tree, often the Japanese emperor oak or daimyo oak.
槲 combines 木 (tree) with 斛, which likely contributes the sound コク. The character was created to name a specific oak species.
The tree radical 木 beside 斛 (a measuring vessel) suggests an oak tree sturdy enough to be measured for timber.
For コク, imagine a cook using oak wood to smoke food: cook -> コク, and the oak tree provides the flavor.