From Quake Wiki
Ogres are the most common foe in the realms of Quake. With a deadly chainsaw for melee fights and a grenade launcher otherwise (though their grenade launcher is weaker than the player's), they can truly pack a punch. If you can, get somewhere higher up than they are, because they are unable to aim upwards and their own grenades might bounce back and hit them. Use a Double-Barrelled Shotgun at close range, 4 shots should put them down. Otherwise, use a Grenade Launcher or a Rocket Launcher for an even return of ammo, as they drop two rockets upon their demise.
The monster_ogre_marksman entity produces a slightly different ogre (won't make a sound when the player is sighted, won't make chainsaw dragging sounds, won't make a proper obituary message, has a different decision-making process when choosing whether to attack, and will in-fight with regular ogres).
monster_ogre is the entity for placing Ogres in a level.
There is also an entity called monster_ogre_marksman, which can also be used to place Ogres in a level. There is no functional difference between the two, but due to them being treated as separate monster types, they can be made to fight each other.
|targetname||The targetname of the Ogre. When triggered, the Ogre will wake up if inactive.|
|target||The targetname of the entity to be triggered when the Ogre dies.|
|killtarget||The targetname of the entity to be removed when the Ogre dies.|
The monster_ogre entity supports the following spawnflags:
|1||Ambush||The Ogre will not wake up from seeing other monsters wake up or hearing sounds.|
It also supports the standard spawnflags for difficulty and deathmatch presence.
|256||Not on Easy||The Ogre will not spawn on Easy difficulty.|
|512||Not on Normal||The Ogre will not spawn on Normal difficulty.|
|1024||Not on Hard||The Ogre will not spawn on Hard difficulty.|
|2048||Not in Deathmatch||The Ogre will not spawn in Deathmatch mode.|
This is the format used by TrenchBroom and Worldcraft. Note that the model(...) parameter is not supported by Worldcraft. This does not include the Monster base class definition, which is also required.
@PointClass base(Monster) size(-16 -16 -24, 16 16 40) model(":progs/ogre.mdl") = monster_ogre : "Ogre" 
This is the definition format used for most old Quake editors, including the original QuakeEd.
/*QUAKED monster_ogre (1 0 0) (-16 -16 -24) (16 16 40) Ambush */