OHCI supports USB 1.1 full and low speed devices. It does not support USB 2.0 or "high speed" devices.
Relevance for Booting
The OHCI driver is crucial for booting.
The OHCI driver is part of the Critical Device Database(see hivesys.inf). That means, if an OHCI device is detected in real hardware then the driver is loaded. But if the driver does not work correctly then the boot process often fails.
The path for the source code of usbohci.sys is /reactos/drivers/usb/usbohci/.