Why does Proto-Slavic first-person singular present form have nasalized o-sound (ǫ) while PIE have longed o-sound?
The thematic verbs in Proto-Slavic had the 1st p. sg. present suffix *-ăm > *-ŏn > *-ǫ whose origin is really obscure. The most probable source of this suffix is by analogy with the athematic verbs which in Proto-Slavic retained the PIE suffix *-mĭ, cf. Proto-Indo-Aryan *Hásmi "I am", Ancient Greek εἶμῐ (eîmi) "I (shall) go" (from Proto-Hellenic *éimi), so in Proto-Slavic it is also *-mĭ > Old Church Slavonic -мь [mʲĭ]: есмь (jes-mĭ) "I am", дамь (da-mĭ < *dad-mĭ) "I shall give".