If the holder is infatuated, it consumes the Mental Herb and is cured of infatuation.
If a Pokémon with Oblivious and holding a Mental Herb is infatuated, Oblivious will cure the Pokémon of infatuation before the Mental Herb has a chance to be consumed.
If a Pokémon with Oblivious and holding a Mental Herb is infatuated or affected by Taunt, the Mental Herb will be consumed before Oblivious cures the Pokémon of infatuation or the Taunt. If a Pokémon holding a Mental Herb triggers Cursed Body or Cute Charm of multiple Pokémon with a single attack, the Mental Herb will be consumed immediately after the first Ability activates, so by the time the second Ability activates the holder no longer has a Mental Herb.
Generations III and IV
If the holder is infatuated, it consumes the Mental Herb and is cured of infatuation.
If a Pokémon with Oblivious and holding a Mental Herb is infatuated, Oblivious will cure the Pokémon of infatuation before the Mental Herb has a chance to be consumed.
Generation V onward
If the holder is infatuated or affected by Taunt, Encore, Torment, Heal Block, or Disable (including from Cursed Body), it consumes the Mental Herb and is cured of all these effects.
If a Pokémon with Oblivious and holding a Mental Herb is infatuated or affected by Taunt, the Mental Herb will be consumed before Oblivious cures the Pokémon of infatuation or the Taunt. If a Pokémon holding a Mental Herb triggers Cursed Body or Cute Charm of multiple Pokémon with a single attack, the Mental Herb will be consumed immediately after the first Ability activates, so by the time the second Ability activates the holder no longer has a Mental Herb.