Cobbled together from the remains of various defense robots by Efi Oladele following Doomfist's temper tantrum at the Numbani airport, Orisa is the latest addition to Overwatch's roster. Taking on the appearance of a robotic centaur, Orisa is a Tank, specializing in laying down suppressing fire, shields, and buffs, and performing crowd control duties. With 200 health and 200 armor, Orisa is fairly resistant to damage on her own, but with the right team, she can become more than just a robot that is designed to merely protect.
Orisa's main weapon, the Fusion Driver, is essentially a toned-down version of Bastion's chaingun. With its fast projectile speed, 200 round magazine, and a rate of fire of roughly 12 rounds per second, it is a very versatile and persistent weapon that is capable of harassing every other Hero in Overwatch (even Tracer and Pharah, if needed), although its damage is rather lackluster, capable of doing only 12 damage per shot (on the bright side, it can headshot people). It may not have much recoil, but its relatively low damage per shot hampers its effectiveness by giving enemies ample time to react and get to cover.
As one of Overwatch's Tanks, Orisa also has a variety of defensive abilities. Fortify gives Orisa a damage resistance buff for four seconds, much like Bastion, effectively doubling her health for the duration while also rendering her immune to any "action-impairing effects." This means that she can resist Reinhardt's Charge (note that this also damages Reinhardt) and Earthshatter, McCree's Flashbang, Winston's Primal Rage, and pretty much any other ability that is designed to either move you involuntarily or stop you from moving at all. Any damage that such abilities apply still affect Orisa, however, so while it may sound like a good idea to use Fortify during Roadhog's Whole Hog, it really just results in you taking all of the damage that the ability would normally do. Orisa can also throw down a protective barrier—think of it as a mini-Reinhardt shield, but deployed like Winston's bubble—which has 900 health and is large enough to comfortably protect any two Heroes.
No Tank is complete without a crowd control ability, and Orisa is no exception. Her Halt! ability functions as a much less impressive version of Zarya's Graviton Surge, capable of dragging people to a different location so that other Heroes can just shoot them to death. That being said, it is not a substitute for Graviton Surge; it is far more effective for simply exposing a sneaky Lucio for a couple of seconds or making people fall off of bridges than anything else.
On paper, Orisa also has one of Overwatch's more mundane Ultimate abilities: Supercharger simply deploys a large canister that buffs the damage of teammates who stand near it. In practice though, it can be very helpful if you need to hold a point. Its radius is large enough that your entire team won't get decimated by Pharah if you're using it, it can be placed on a payload to provide a mobile damage boost, and it provides 15 seconds of 50% extra damage. Unfortunately, it can be destroyed by gunfire, and it obviously makes your team much more susceptible to things like Graviton Surge and Blizzard, but it synergizes particularly well with Lucio and Zenyatta's Ultimate abilities.
When all is said and done, however, where does this leave Orisa in your average Overwatch match? She can't replace Reinhardt since her shield is nowhere near as resilient, she lacks Roadhog's ability to displace and kill someone virtually instantly, and her low movement speed means that she isn't going to be filling D.Va or Winston's role in high mobility teams anytime soon. Instead, she will have to function as Reinhardt's backup tank, throwing down shields whenever Reinhardt's shield breaks and laying down a continuous stream of fire while the team is advancing behind his shield. Another Reinhardt feels like charging your Reinhardt? Try to intercept him with Fortify. Roadhog wants to drag someone from behind a shield? Crush his dreams with a shield, or try to at least. This does mean that she will have to compete with Zarya or Roadhog, though if your team isn't using Reinhardt anyways this shouldn't be much of a problem. Perhaps as time goes on and people become more familiar with her abilities, Orisa will be able to fulfill frontline Tank roles more effectively, but for now it would probably be for the best if she acts as more of a Support-Tank hybrid.