From olúwa ("Lord") + ṣe ("do") + oun ("thing"): "the Lord has done (something good)." Also read as "thank God," since ẹ ṣeun is the everyday phrase for thank you. Shortened to Seun.
Source: Yoruba olúwa- theophoric naming, cross-referenced across yorubaname.com (Olúwáṣeun, full morpheme breakdown) and independent Yoruba-naming guides