Contrary to popular belief: Ohmwrecker isn't a bad item but tricky one
The item itself has a normal stat value (being 101% gold efficient) and it has a very High winratio. Now of course this is mainly due to the fact that it's picked up rather rarely and most people who pick it up know what to do with it.
Now for situations to pick it up: It's actually pretty strong during every gamestage. In early it allows towerdives while in late it allows you to take down a turret quickly without taking too much damage. The main problem are the stats and the usage.
The item provides a rather small amount of ability power and a good chunk of health. So basically you should only pick it up on AP-Supporters like Sona, Nami, Janna etc who benefit from both stats. You can also sometimes buy it on junglers like Amumu or Sejuani since a dive with these champions while the turret doesn't attack will most likely result in a kill.
This one is pretty tricky. Turrets deal increased damage for every hit they've already landed. Ohmwrecker resets this bonus damage. So basically you should try to soak up 2 hits and then use the item as soon as the 3rd is about to go off. with this you have about 5-7 seconds to kill the enemy which should be fairly easy if you have enough CC.
So basically: It can't hurt to pick up the item on an AP supporter, since there aren't too many other important item that would fit into this slot. Maybe try to get your Crucible and Sightstone first but if you're ahead and you want to towerdive you can also go for an Ohmwrecker since it's fairly cheap.