As already mentioned by ewwhite, the
textcons mode can allow you access to text-mode display. With Linux, this means you can access the text console if X11-based GUI login has been disabled, or if you use boot parameters to force the system into single-user/emergency mode.
But since the
textcons display is based on scraping the memory of the VGA controller, and the keyboard input requires mapping the characters sent over SSH back to scancodes which the OS will then re-translate to characters, this can be clunky and error-prone.
In Linux and the *BSDs, OpenSolaris and the like, you can configure a
getty process on any serial port to allow traditional Unix terminal-based login - this includes the virtual serial port provided by iLO. With the
vsp command, you can access the virtual serial port through a SSH connection to iLO. GRUB bootloader can also be configured to communicate over a serial port, instead or in addition to the regular VGA console access. And on server systems with a iLO or similar remote console, the system firmware can often be also configured to send its boot messages to the serial port - either automatically when a connection is detected, or by changing a BIOS setup option.
As a result, with appropriate preparations, you can have full control of a system over a SSH connection to iLO: first you can see the firmware boot messages and even make changes to the BIOS/UEFI setup, although the interface might not necessarily be the same as on a VGA console. Then, with appropriate configuration, you can command GRUB over the VSP virtual serial connection too. And if you use a boot parameter to switch the Linux console to a serial port, or set up an extra
getty process, you can even log in to the OS.