Usage patterns
木の洞に〈物〉を隠すto hide something in a hollow of a tree
I hid the acorns in the hollow of the tree.
洞ができるa hollow forms (in a tree, rock, etc.)
A hollow had formed at the base of the old tree.
Expressions
洞ヶ峠を決め込むwait and see which way the wind blows; sit on the fence; wait for a good opportunity