Is there a respec mechanism in Dragon Age: Origins?

If I remember well it is not possible respec characters in game without mods.

But there is a really nice mod, called Character Respecialization.

This addin allows the player to reset the base attributes, specialization points, spells, talents and skills of the hero character and any of the party members to the default values and returns the remaining points so they can be spent again.

The official Dragon Age Nexus mod page provides detailed instructions. Just a hint: do a backup of your profile before to ensure you do not accidentally lose or corrupt it.

Related to this, in Dragon Age: Origins Awakening, they have an item called the Manual of Focus, which can be purchased from Herren in the Vigil's Keep courtyard. It costs 6 gold to use and you can use it as many times as you want on any character, as long as you have the money for it. So if you decide to import your DA:O character into Awakening, you can also respec them that way without a mod.