Ἐνταῦθα λέγεται ̓Απόλλων ἐκδει̂ραι Μαρσύαν νικήσας ἐρίζοντά οἱ περὶ σοφίας (Xenophon, Anabasis 1.2.8)

[amhlaoibh]

I have a question about a passage in Xenophon (Anabasis 1.2.8):

ἐνταυ̂θα λέγεται  ̓Απόλλων ἐκδει̂ραι Μαρσύαν νικήσας ἐρίζοντά οἱ περὶ σοφίας

I'm really thrown for a loop by the οἱ in the phrase.  I parse the sentence as something like:

There it is said Apollo (nom sg.) flayed (aorist infinitive) Marsyas (acc. sg.)
having conquered (aorist participle nom. sg.) him competing (present participle, acc. sg.) ..

The last three words come out something like "these concerning skill".  But I don't understand what οἱ is doing here.  That is a nominative plural, but I can't see how that fits in. 

[wings]

For those who can help, the whole quote from Anabasis, 1.2.8, is:

[1.2.8] ἔστι δὲ καὶ μεγάλου βασιλέως βασίλεια ἐν Κελαιναῖς ἐρυμνὰ ἐπὶ ταῖς πηγαῖς τοῦ Μαρσύου ποταμοῦ ὑπὸ τῇ ἀκροπόλει· ῥεῖ δὲ καὶ οὗτος διὰ τῆς πόλεως καὶ ἐμβάλλει εἰς τὸν Μαίανδρον· τοῦ δὲ Μαρσύου τὸ εὖρός ἐστιν εἴκοσι καὶ πέντε ποδῶν. ἐνταῦθα λέγεται Ἀπόλλων ἐκδεῖραι Μαρσύαν νικήσας ἐρίζοντά οἱ περὶ σοφίας, καὶ τὸ δέρμα κρεμάσαι ἐν τῷ ἄντρῳ ὅθεν αἱ πηγαί· διὰ δὲ τοῦτο ὁ ποταμὸς καλεῖται Μαρσύας.

[billberg23]

Oἱ here is not the masculine plural nominative article.  Instead, it is a brief enclitic alternative to the dative reflexive ἑαυτῷ, "to/for himself."  So "competing with himself (i.e., Apollo) over (musical) skill."  In early inscriptions this dative oἱ appears as Foι, with its original digamma.