Where does “ō” come from?

ō (Old English) comes from Latin o, from Etruscan 𐌏, from Ancient Greek ο, from Phoenician 𐤏, from Egyptian 𓁹.

Ancestry of “ō”, step by step

Step	Language	Word	Meaning
1	Latin	o	The name of the letter "O"; o!; oh!
2	Etruscan	𐌏	—
3	Ancient Greek	ο	Lower-case omicron (ὂ μικρόν), the 15th letter of the ancient Greek alphabet. It represented the close-mid back rounded vowel. It is preceded by ξ and followed by π
4	Phoenician	𐤏	ayin; ʿayin
5	Egyptian	𓁹	—