Two is Better Than One

Spam, spam, spam

Ogre Magi's ult is more useful the more you use your skills. His primary skill is the fireblast, which can deal a large burst of damage as well as stun an enemy for 1.5 seconds. Sadly, for an intelligence hero, Ogre's int and mana pool are sub par, on the other hand his str and hp are better than many strength heros'. Because of this, it really helps to get stats at levels 2 and 4 to maintain your mana pool, and it is recommended that you don't waste your mana by using the level 1 fireblast, as the damage is laughable. Once you can get the ul, get it, then get a level each of ignite and bloodlust, and then max bloodlust followed by ignite.

As for the items, you should have 100 gold left over after the initial purchase, so getting a magic wand will only take a few last hits and it will free up precious inventory space as well as give you extra mana when you need it. Getting a soul ring should be your second priority unless you are being harassed and need some speed, in which case get boots of speed first and then go right for the ring. 150 is roughly how much a fireblast costs(depending on its level, your ult level), and 150 hp isn't very much for Ogre, especially when you add the 3hp/s regen from the ring and later 3hp/s more from the boots. So whenever you have more than 150 mana missing and enough hp to safely be near enemies, use the ring just before fireblasting, and if you have tranquil boots, use them to heal yourself right back up. This way your mana will last you much longer, and you will be able to get more fireblasts off. And more fireblasts means more multicasts.

Aghanim's is a very useful item for Ogre as it gives him his primary spell again, but with a larger mana cost and a longer cooldown. Because of this it is recommended that you get Eul's first for the extra int and mana regen, as well as the cyclone. After that you should have enough mana to, if not spam, then at least not instantly run out of mana after casting both fireblasts.

Luxury items are anything that increase mana regen or increase magic damage. Heart can be useful too, to make killing you all the more difficult.