Tidings - this is a short report that the Crafted set Oblivion's Foe only provides a boost to Weapon Critical in-game.
This contrasts with the patch notes from Update 28 which stated that the 4-piece bonus on the set provided "833 (now 857) Critical Chance."
Critical Chance is the new unified bonus that has been appearing on the most recently introduced or updated Crafted sets. It also fits in with the unified bonus theme of the overall set, which provides unified bonuses to Penetration and Weapon/Spell Damage.
Another set with the unified Critical Chance bonus, Legacy of Karth, is working as intended however. So this is a bug limited to Oblivion's Foe.