Ojek “motorcycle taxis” began appearing in Jakarta after becak were banned in 1994. Ojek service began as a people's initiative to provide a transportation options for people who used to use becak from main roads into housing complexes. There is no government licensing for or control over ojek.
By law all motorcycle passengers should wear helmets, so ojek drivers should have a spare for you to wear. Ojek tend to congregate at t-junctions on main roads and near smaller roads that are not serviced by bus routes. Ladies have a careful balancing act if wearing a dress and must sit sidewise on the back of the vehicle.
Bargain before you get on - ask a local what the price should be first.