OHCI
OHCI (Open Host Controller Interface) is a standardized interface for the connection of an USB 1.1 host controller to the PCI bus. The driver for OHCI in ReactOS is usbohci.sys.
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.
Source code
The path for the source code of usbohci.sys is /reactos/drivers/usb/usbohci/.
For high level functionality of the driver the libusb is used (reactos/lib/drivers/libusb). Thus changes in libusb influence the functionality usbohci.sys.
