رَأَيْتُ عُمَرَ — I saw ʿUmar.
رَأَيْتُ: past-tense verb built on sukūn because it is attached to the subject tāʾ («تُ»). The «تُ» is the subject pronoun.
عُمَرَ: direct object, accusative, visibly marked by fatḥa.
Its accusative status follows from its role as the object of the verb, rather than from the vowel considered alone.