Sounds like this is the official series of events:
9:45 or so this morning, a fire alarm was pulled in a watt hall lab for a possible chemical leak. Fire department had cleared the building for re-entrance when a car came speeding around the corner and went on to a sidewalk, hitting multiple people before hitting a brick planter. As people rushed to the car to see if the driver was okay, the suspect exited the vehicle with a butcher knife and began to stab people. At this point, police believe the timing with the fire alarm was a coincidence.
An OSU Police Officer was on site because of the chemical/gas leak and responded within 1 minute of the attack. The officer engaged the suspect and fired three shots. At that point, the active shooter alert went out to campus.
There were some conflicting witness statements about how many people had exited the car, but surveillance video was surrounding areas show that the suspect was alone in the car. Some witnesses said another potential suspect fled to a nearby parking garage. Since the alert told students to "Run, Hide, Fight", many other students decided to hide in the garage. This is why we have reports of anywhere of 2-3 people being seen handcuffed. Police were securing the garage.
After searching a few other buildings, the alert was lifted at around 11:30 AM, almost 90 minutes after the event was over. 11 people in total were injured, most were injured by the car and 2 were stabbed. One victim has a fractured skull.
FBI and BATFE are still on site and helping with the investigation. The suspect was a legal resident of the US from Somali. He was apparently a former student at OSU and some recent Facebook activity suggests that there may be terrorism ties.