Almost all Doom cheat codes will work on Doom Online, since it’s a port of the original shareware to flash.

And here they go:

IDDQD: The God Mode
IDKFA: All Weapons, Full Health and All Keys
IDFA: All Weapons, Full Health without the keys
IDSPISPOPD: No clipping mode
IDDT: Revealing the entire map in map mode
IDBEHOLD: Picking up Additional Items. Use like IDBEHOLD + S (berserk), V (invunerability), I (invisibility), R (anti radiation suit), A (map computer) or L (light amplificator);
IDMYPOS: Your position in HEX code

Source: Códigos de Trapaça do Doom