Yes another Synthesist bugging problem!

[lordlor]

If someone was to dual class with an oracle taking the Lore Mystery, they use Revelation:
Sidestep Secret (Su): Your innate understanding of the universe has granted you preternatural reflexes and the uncanny
ability to step out of danger at the very last second. Add your Charisma modifier (instead of your Dexterity modifier) to your Armor Class and all Reflex saving throws. Your armor’s maximum Dexterity bonus
applies to your Charisma instead of your Dexterity.

But does this when merged with the eidolon over ride the eidolon's own dexterity??
It is a supernatural ability remeber

[nicklas Læssøe]

If you do indeed multiclass and gain the ability then yes.

Remember that u dont actually override dexterity, you still use the eidolons dex to determine stuff like skills and initiative, but with regards to armor class then yes it would indeed function like that.

[Nephril]

agree with nicklas you would be able to utilize the ability. your eidolons dex would still be there you just would use force of personality to increase your ac instead of nimble moves.