Sentence:  Three <WSD>o'clock</WSD> and nine <WSD>o'clock</WSD> would be at the outer right and outer left sides of the tree, respectively, and so on. The clock technique is a very helpful way for one bird watcher to direct others to a hard-to-spot perched bird. 
Meaning of the word marked with <WSD> tags:  In conjunction with a numeral, indicates the direction, relative to the speaker or a vehicle, especially an aircraft, corresponding to the direction the hour hand is pointing at the time corresponding to the numeral, with twelve representing directly ahead and three being to the right.