The Oracle will give you several clues.  Not all of them, according to the manual, will be the correct ones.  Only way to find out for sure is to experiment.  Below is a list of the clues I have gotten from the Oracle so far.  I am always finding new ones so I will update this area when I discover new info:
'In order to learn the Secret of the ... you must possess the ...'
(1) SCORPIAN -->  (a) Skull, CROM  (b) Flute, NERGAL

(2) TROGLODYTE -->  (a) Spellbook, CROM

(3)  VIAL -->  (a) Talisman, MITRA  (b) Mushroom, VETAR

(4)  POWERRING -->  (a) Shield, Mace, Skull, MITRA  (b) BELROG

(5)  SPRITE -->  (a) Dagger, AKHIROM  (b) Food, NERGAL  (c) Skull, Powerring

(6)  MINOTAUR  -->  (a) Ax, NERGAL

(7)  TRUTHRING -->  (a) Lightring, CROM, Basket  (b) Lightring, Ax, OKKAN

(8)  LIGHTRING -->  (a) Powerring, Rope, NERGAL  (b) Powerring, Ax, OKKAN

(9)  SWORD  -->  (a) MITRA  (b)  Rope

(10) SKULL  -->  (a) Scepter

(11) SATYR  -->  (a) Spellbook, VETAR

(12) NYMPH  -->  (a) Mushroom, Truthring

(13) SHIELD  -->  (a) Mace