Full parsing: Abū Ḥafṣ ʿUmar

ʿAṭf al-Bayān: Definition, Examples, and Contrast with Badal

قَامَ أَبُو حَفْصٍ عُمَرُ.
قَامَ: past-tense verb, built on fatḥ.
أَبُو: nominative subject with wāw, one of the six nouns; also muḍāf.
حَفْصٍ: genitive muḍāf ilayh with kasrah.
عُمَرُ: nominative explanatory follower with ḍammah.

«عُمَرُ» shares the nominative case of «أَبُو», but not its visible marker: wāw versus ḍammah.