Ogre Magi is a pretty tanky ganker and support hero. He can also roam pretty well with an early set of boots (not starting item, but early) because he has a reliable stun and slow. Situationally, he can also do decent at mid.
- 2 x Clarity
- Courier / Wards
- 1-2 x Iron Branches
Alt Start (it's a bit unorthodox, but I like it. Don't do this if you're in lane with a carry though or you are the primary support):
- Arcane: This is the standard choice as its good for his mana pool, works as a support item, and he needs the mana to spam.
- Phase: If you have an alternate source of mana (e.g. Soul Ring, Bottle), Phase boots can be picked up as they give him good speed to gank between lanes and chase.
- Magic Wand always a good item, especially good if they have a spammy team
- Force Staff: because it's good in so many situations. You can use it to chase, escape, save an ally, screw an enemy, etc.
- Support items (Wards, Dust, Smoke, etc..)
- TP Scrolls: Need these for ganking, counter-ganking, and defending.
- Soul Ring: If you need an alternative source of mana, good for ganking where you're getting lots of kills, etc.
- Bottle in the scenario you are mid.
- Vanguard: If you want to build up your tankiness. One of the most annoying things is a support that's hard to kill. Especially since OM gives a stun, a slow, and makes the entire team more effective with Bloodlust.
- Mekansm: Solid support item.
- Urn of Shadows if you're ganking a lot.
- Orchid Malevolence If you're doing well enough to farm one up, the extra silence can be very useful, as well as the mana regen. This is especially useful if they have a blink hero (e.g. Anti-Mage, Queen of Pain) so that they cannot blink away as easily.
- Eul's Scepter of Divinity: Good for the mana regen, the extra CC, and the movespeed.
- Shiva's Guard: Good for the extra armor and INT. The active is powerful for teamfights. You're also very tanky, so the aura is effective on you.
- Pipe of Insight: If no one else has one, you're a pretty good carrier.
- Ghost Scepter: If they have a lot of physical damage and you can't handle it.
- Drum of Endurance: Good pickup if you're behind or no one else has gotten one.
- Veil of Discord: More damage from your spells. Also, the stats aren't bad for OM.
- Aghanim's Scepter: Gives you an extra fireball. Great for Crowd Control and the stats.
- Bloodstone: Gives you increased ability to spam.
- Scythe of Vyse: More crowd control, good stats.
- Blink Dagger: Great for chasing, good for jumping in to surprise the shit out of your enemy.
- Ethereal Blade If you're balling out of control, dunking bitches left and right, and want to upgrade your Ghost Scepter to the point of nearly 1 shotting anyone with a lucky multi-cast with that sweet 40% extra magic damage.
He's pretty straight forward to build.
- Prioritize Stun
- Second priority is Slow
- Take Ult when available
- Level Bloodlust last
- Early game you're a very aggressive lanemate
- You roam and gank pretty well
- Ideally paired with another stunner
- Late game, you're good for the crowd control and your Bloodlust steroid spell. Once you have the regen, you should be constantly casting Bloodlust to keep your team at maximum potential.
- You can be a pretty dangerous mid if you're in the right lineup. OM is good at ganking and early levels give him more magic damage.
- If you tank up a bit, then you become super annoying since most people want to focus support. Adding to his natural tankiness means the enemy spends longer on you and not attacking your DPS.
- You're relevant basically the entire game. Early game, you have powerful nukes and crowd control. Mid game, those skills are maxed out and your multi casts help you decimate. Late game, you become full support basically due to having crowd control and Bloodlust to aid your teammates.