From olú ("God, Lord") + ṣẹ́gun ("conquer," itself ṣẹ́ "do" + ogun "war"): "God is victorious" / "God has conquered." Also spelled Oluwasegun. Shortened to Segun.
Source: Yoruba olú- theophoric naming, cross-referenced across Wikipedia (Olusegun), wisdomlib, and independent Yoruba-naming guides