Where does “ѥлѣи” come from?

ѥлѣи (Old Church Slavonic) comes from Byzantine Greek ἔλαιον.

Ancestry of “ѥлѣи”, step by step

Step	Language	Word	Meaning
1	Byzantine Greek	ἔλαιον	—

ѥлѣи — Etymology, Origin & Word History | Etymology Explorer