Who was the King Okkāka mentioned in Sutta Nipāta?

Check out Pali Dictionary of Proper Names:

Okkāka

A king, ancestor of the Sākyas and the Kolians. In the Ambattha Sutta (D.i.92) it is stated that Okkāka, being fond of his queen and wishing to transfer the kingdom to her son, banished from the kingdom the elder princes by another wife. These princes were named Okkāmukha, Karakanda, Hatthinika, and Sīnipura.

The Mahāvastu (which confuses Iksvāku with his ancestor Sujāta) mentions five sons of Iksvāku: Opura, Ulkāmukha, Karandaka, Hastikasīrsa and Nipura (i.348). See also Rockhill, p.9ff.

They lived on the slopes of the Himalaya and, consorting with their sisters and their descendants, formed the Sākyan race. The legend, thus briefly given, is enlarged on with great detail in the Commentaries. According to Buddhaghosa, there are three dynasties with a king named Okkāka at the head of each, all of them lineal descendants of the primeval king, Mahāsammata, and in the line of succession of Makādeva. The Okkāka of the third dynasty had five queens - Bhattā, Cittā, Jantū, Jālinī and Visākhā - each with five hundred female attendants. The eldest queen had four sons - mentioned above - and five daughters - Piyā, Suppiyā, Anandā, Vijitā and Vijitasenā. (The Mtu. calls them Suddhā, Vimalā Vijitā, Jālā and Jālī).

When Bhattā died, after the birth of these nine children, the king married another young and beautiful princess and made her the chief queen. Her son was Jantu, and being pleased with him, the king promised her a boon. She claimed the kingdom for her son, and this was the reason for the exile of the elder children (DA.i.258f; SnA.i.352f). The Mahāvamsa (Mhv.ii.12-16) mentions among Okkāka's descendants, Nipuna, Candimā, Candamukha, Sivisañjaya, Vessantara, Jāli, Sīhavāhana and Sīhassara. The last named had eighty-four thousand descendants, the last of whom was Jayasena. His son Sīhahanu was the grandfather of the Buddha. The Dīpavamsa (iii.41-5) list resembles this very closely.

Okkāka had a slave-girl called Disā, who gave birth to a black baby named, accordingly, Kanha. He was the ancestor of the Kanhāyanas, of which race the Ambattha-clan was an offshoot. Later, Kanha became a mighty sage and, by his magic power, won in marriage Maddarūpī, another daughter of Okkāka (D.i.93, 96).

According to the Brāhmana-Dhammika Sutta (Sn.p.52ff; AA.ii.737), it was during the time of Okkāka that the brahmins started their practice of slaughtering animals for sacrifice. Till then there had been only three diseases in the world - desire, hunger and old age; but from this time onwards the enraged devas afflicted humans with various kinds of suffering.

It is said (DA.i.258) that the name Okkāka was given to the king because when he spoke light issued from his mouth like a torch (kathanakāle ukkā viya mukhato pabhā niccharati). Although the Sanskritised form of the Pāli name is Iksavāku, it is unlikely that Okkāka is identical with the famous Iksavāku of the Purānas, the immediate son of Manu, son of the Sun. The Pāli is evidently more primitive, as is shown by the form Okkāmukha, and the name Iksavāku looks like a deliberate attempt at accommodation to the Purānic account. For discussion see Thomas, op. cit., p.6.

According to the Mahāvastu, Iksavāku was the king of the Kosalas and his capital was Sāketa - i.e. Ayodhyā. See also s.v. Sākya.

The Cūlavamsa mentions among Okkāka's descendants, Mahātissa, Sagara and Sāhasamalla (q.v.).

2. Okkāka.

King of Kusāvatī in the Malla country. He had sixteen thousand wives, the chief of whom was Sīlavatī. As a result of her consorting with Sakka, two sons were born, Kusa and Jayampati.

The story is related in the Kusa Jātaka. J.v.278ff.