Instead of just turrets?
Turrets can be disabled for a short time, with little to no change.
Inhibitors can temporarily take increased damage, or it can just be a nuke that scales with defensive stat. Or a DoT that does damage so long as the inhibitor remain in combat.
Nexus should probably not have any interactions with the item.
Optionally, maybe the base gates can be temporarily be opened to both teams upon the use of Ohmwrecker.
The idea is that Ohmwrecker will become the way to break into the enemy base when they are turtling.