These are the only mobs that can destroy blocks. They smash with a level of strength that varies according to each type of Ogre. The Ogres are twice the size of the player, and are very dangerous. However, they can quickly be killed with a bow and arrow at a safe distance or taken on rather easily with a simple sword as long as you have a good location to do it. Make sure to keep moving backwards or jumping backwards because it seems to work alright. Be wary of collateral damage.
Ogres seem to spawn near villages at night, often accompanied by a werewolf or other Ogres. They will also spawn randomly around the world in the same fashion or on their own. To lessen the danger of Ogre destroying your house, build it out of basalt or Marble brick, as Ogres can not destroy it. However, make sure you place it down far enough that they can't get underneath your wall and destroy your floor if the floor is made of another material and it wil subsequently destroy any chests or objects you have placed. In addition Ogres can destroy objects like beds through a single block wall so you may want to make your house with thick walls. Any object that is "dropped" on the ground can be destroyed permanently by an Ogre. Basalt Cobblestone has now been tested and confirmed to be Ogre proof, therefore saving a decent amount of fuel.
Common Ogres drop Obsidian upon death unlike the Cave Ogres, which drop diamonds. Common Ogres are 4 blocks high. The attack range of a Common Ogre is 5 blocks high and three blocks forward. They are a deep shade of green (pictured).
Fire Ogres have a bigger area of effect than the Green Ogre, and can sometimes cause nearby blocks to catch on fire when they smash. Upon death, the Fire Ogre drops Fire, providing yet another way of making Chain Armour with the Mo' Creatures mod and overall the entire Technic pack.
Cave Ogres only spawn underground and have the biggest AoE of all the Ogres. They sometimes drop Diamonds, making them almost twice as useful as the Green Ogres.