Just going by the Greyhawk gods of the PHB, I'd go with:
Boccob = Rad
Corellon Larethian = Ilsundal
Ehlonna = Djaea
Erythnul = Alphaks
Fharlanghn = Odin, maybe?
Garl Glittergold = Garal Glitterlode
Gruumsh = Karaash
Heironeous = Halav
Hextor = Atzanteotl
Kord = Thor seems the closest
Moradin = Kagyar
Nerull = Thanatos
Obad-Hai = Zirchev (maybe)
Olidammara = Asterius
Pelor = Ixion
St. Cuthbert = Erg. Really not sure here. His stance with regards to the law and justice seems like either Forsetta or else Tarastia.
Vecna = I don't think Vecna's really got a good counterpart. If I had to pick one, I'd probably go with Loki.
Wee Jas = Hel (possibly)
Yondalla = Tough one, as there aren't really any "halfling" specific Immortals. Her portfolio sounds a bit like Petra, though.
I'll look through my other Greyhawk deities in a bit and see if I can come up with some other correspondences.
Question, though- how direct a match are you looking for? Just ideology? Or Ideology plus Power Level plus Gender? As seen above, Tarastia seems (to me) an ideological counterpart to St. Cuthbert, but they're different genders. And Boccob, IIRC, is a lot more powerful than Rad should be.