The genitive signals an exclamation, usually preceded by an interjection.
Common interjections include φεῦ and οἴμοι ‘alas!’.
In tragedy the genitive after οἴμοι or ὤμοι ‘alas!’ refers to the second or third person. For the first person the nominative is used.
According to most grammars (including Rijksbaron and others 2003) this genitive has a causative role.