You can get a complete list of available keyboard commands by visiting the Key Bindings page of the Orca Configuration GUI. To bring up the Orca Configuration GUI, press Orca_Modifier+Space, where Orca_Modifier is the Insert key for the desktop layout and the Caps_Lock key for the laptop layout. Note that you can also enter Orca's "learn mode" while running Orca by pressing Orca_Modifier+H. When in learn mode, Orca will intercept all keyboard and braille input events and tell you what the effect would be. To exit learn mode, press the Escape key.
Orca provides additional commands for some applications, such as Firefox. To get the list of additional commands for an application, press Ctrl+Orca_Modifier+Space when that application has focus. This will bring up the Orca Configuration GUI in the "application-specific" state, and the additional commands will appear in the Key Bindings page.
The GNOME Desktop itself also has built in keyboard commands to control the desktop and its applications.
WARNING: the MouseKeys functionality of AccessX can interfere with the Orca modifier key (see bug 548169), resulting in odd behavior. In particular, MouseKeys uses the KP_Insert key to emulate a mouse button press. When enabled, MouseKeys can conflict with KP_Insert as the Orca modifier, resulting in what appears to be a stuck Orca modifier key. As an alternative, you can use the laptop key bindings, which avoids the keypad for Orca commands. If you accidentally enabled MouseKeys, you can disable it via the gnome-keyboard-properties application.