This page details various suggestions on how to defeat ogres in Dragon Age: Origins.
Strategy
Battling an Ogre can prove to be a somewhat troublesome endeavor, as they possess considerable defensive and offensive capabilities, are difficult to effectively flank, and can quite easily kill party members with inadequate armor; fighting them is especially difficult in the beginning of the game, when party members will lack a great deal of versatility that would otherwise aid in slaying this towering Darkspawn.
The first ogre
Considering the low levels and lack of talents/spells of the team, the first ogre to face the player in the Tower of Ishal can be one of the toughest fights in the game. Yet with a simple tactic this fight can be one of the easiest. First, scatter your team to four different directions as soon as the cutscene ends, and observe which one the ogre goes after. Second, take control of that character and have him or her lead the big lumbering beast on a merry chase along the circumference of the circular room. Since the ogre has the same speed as you it will never catch up to you. Meanwhile send the other three to the center of the circle, equip ranged weapons, and open fire. If and when the ogre gives up the chase and charges toward the center, just scatter the group at the center, observe which one it goes after, reorganize the 1-and-3 grouping, and start the process all over again. In the end it's very likely that the ogre will drop dead without landing a single hit on anyone of your team.
Another good strategy is to send all your soldiers to attack, but pausing the game oftenly and see any different movements made by the Ogre. If he moves his arm or even rise his feet, make everyone pull away, since his attacks sometimes have Radius damage. If made carefully, your squad can leave intact.
General strategies
Early on, Ogres should be treated cautiously. They possess formidable physical resistance, considerable amounts of health and armor, and several lethal skills. Typically, they will Ram if they gain the opportunity to preemptively attack the party, which can cause heavy damage and mass knockdown if done by surprise. In general combat, they can perform three significant abilities, Grab, Hurl, and Massive Attack. Grab will cause the Ogre to snatch a party member and suspend them in midair, beating them savagely with its free hand for significant damage over time; this is the Ogre's most lethal single attack, and one should always pay attention to freeing a Grabbed party member. Massive Attack is a universal anti-flank ability for large creatures such as Ogres, Drakes, Dragons, and High Dragons. When used, Massive Attack causes the monster to lash out and strike most or all melee-range characters for moderate damage and a brief knockdown. Finally, Hurl causes the Ogre to tear a sizable chunk of rock from the ground and lob it at the party. The impact causes splash damage and knockdowns, and can severely hamper efforts to stay on even terms with the Ogre. If managed effectively, Ogres are not difficult to bring down; below are a number of strategies to aid in doing so.
With tank
If the party possesses a tank warrior, they should be commanded to fire the first shot of a distanced "pull" to lure the Ogre closer. After the tank's shot has been fired, other party members should begin firing arrows/bolts/staff projectiles at the Ogre to deal as much safe damage as possible. When the Ogre comes within roughly ten yards of the party, the tank should equip melee weaponry and activate some form of threat-evocation (Taunt, Threaten), then proceed to assault the Ogre. Once the tank has entered combat, other applicable party members should attack the Ogre with their primary armaments. Flanking, while certainly an aid to damage, is not as effective as it would normally be due to the Ogre's ability to unleash Massive Attack, hitting most or all melee-range foes with moderate damage and knockdown. If the Ogre does not perform Grab, the battle should be a relatively steady damage race, bringing the Ogre down before it has a chance to significantly harm or even kill party members. In the case of a Grab, especially if the tank is a victim, party members should be ready to use any stun-inducing skills available, as stunning the ogre will usually, but not always, cause it to release the Grabbed character. Other things to keep in mind with a tank party are the use of Shield Wall with Shield Expertise will help greatly by keeping the tank from being knocked down under any circumstances, although Grab is still an issue.
Without tank
Without a tank, battling an Ogre can be a highly dangerous endeavor due to its ability to significantly harm poorly-armored party members with astounding celerity. Without a tank to hold the attention of the Ogre, the best strategy is for a Mage or trap-user to ensure the Ogre is held in place or significantly slowed so as to allow the party to assault it from a safe distance. While this can be risky if the skills being used fail or are resisted, it provides a very easy encounter when done successfully; the Ogre has only one significant distanced attack, whose effects can be mitigated by spreading party members out. Again, when done properly this strategy results in a simple damage race with the Ogre, killing it before it can cause harm. The party will be significantly aided by melee shutdowns, stuns, and various Hexes.
Mid- to Late-Game
With character progression, Ogres become considerably less threatening; by level 14 a tank-talented Warrior player can kill one alone in under forty-five seconds if well-equipped. With a balanced party, common (white name) Ogres are largely trivial, although they can still be hindrances if found in conjunction with numerous other Darkspawn. Ogre Lieutenants (yellow name) are tougher than standard Ogres, but still largely ineffective against an experienced party. Finally, Boss-level Ogres (orange name), can be threatening, especially with parties of accompanying Darkspawn, but should be treated with the same caution as early-game Ogres; while significantly stronger, their advantage is only in level-scaled hit-points and damage, meaning that the tactics for defeating them are unchanged, but will merely take longer.
Solo
This section gives advice on how to deal with Tower of Ishal Ogre. It can be applied to all other ogres the Warden encouters, but later on more spells and talents provide other and probably better solutions.
Mage:
Activate Haste. This way the Ogre cannot catch you. Run around in circles until sufficient gap is created, turn around and hit the Ogre with a spell. Repeat as long as it takes to kill the Ogre. Using normal staff projectile instead of a spell will not work, as the projectile will not be fired while Ogre is running. Hurl projectile can be evaded if you clear out of the impact zone before it hits, so if you see the Ogre picking up a rock, run away as far as possible from the place you were at in the moment that ability was activated.
Rogue:
Run around in circles, the Ogre will follow. When it tries to attack, it will pause for a moment, and create a small gap. Turn around quickly, and activate Dual-Weapon Sweep in the moment just before the Ogre enters the attack zone. If timed correctly, Dual-Weapon Sweep will start while it is out of range, deal damage while in the attack zone, and still give you time to start running away before the Ogre has a chance to hurt you. Remember to deactivate any sustained abilities you may have, they will just slow down your stamina regeneration and increase the cost of activation of Dual-Weapon Sweep.
Warrior:
Dual Weapon warriors might want to use same tactics as Rogues. Weapon and Shield or Two-Handed warriors who do not wish to waste a talent point into Dual-Weapon Sweep do not have any reliable means of staying out of reach. Make sure that Powerful talent has been chosen for extra life. Activate shield wall if available. Wear the heaviest armor for best protection, prepare lots of Health Poultices and a Rock Salve. Even without attribute points spent on constitution, this should give enough life and armor to survive Grab when near full health.
Class-specific tips
If the Player is a Mage, various forms of melee shutdown combined with ranged attacks can make Ogres trivial; Crushing Prison and Misdirection Hex, if successfully cast, can reduce an Ogre to an extremely ineffectual combatant. Alternatively, spells such as Blood Wound or (with great difficulty) Blood Control can serve much the same purpose.
If the Player is a Warrior, stunning skills such as Shield Pummel, Assault, and Riposte can prove invaluable for inducing stuns, while the Shield talents Shield Wall, Shield Expertise, and Shield Mastery can result in the tank being generally unthreatened by the majority of the Ogre's abilities. Ogres are affected by the Dual-Weapon warrior's Punisher talent, and can suffer a knockdown from this ability also.
If the Player is a Rogue, Dirty Fighting provides an excellent emergency stun, while rogue backstabbing talents can aid in melee damage. With skills such as Evasion and talents in the Dual Weapon specialization, Rogues can act as impromptu distractions if a tank is unavailable, although this is highly inadvisable if the rogue's Defense score (not Armor) is low.
Miscellaneous tips
As with all large monsters, Ogres suffer from a severe vulnerability to minefield tactics; a high concentration of damaging traps in a relatively small area can result in severe wounds or even instant death for an Ogre.
Shale and Dog cannot be picked up by Ogres, making their Grab attack useless. However, they can still be hit with every other attack from an ogre. You can also implement the Rogue strategies against an Ogre, using Dog's increased running speed. (He runs two times faster than The Warden.)
As the game progresses, Ogres will be found in pairs, trios, and sometimes larger groups. These encounters can prove unpleasant, but with sufficient healing and a reliable tank, they are not overly difficult. The largest group of Ogres can be found in the game's final quest, consisting of some six Ogres spread over a very brief interval that will result in the party being forced to fight three or more for considerable lengths of time.
As an alternative to using skills for melee shutdown, a character with sufficient Poison-Making could use Deathroot Extract or Concentrated Deathroot Extract to inflict repeated stuns on the Ogre, in addition to the poison's added Nature damage.