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Saiyan Levels
Super Saiyan 1
This stage was first reached by Gokou when Furiza killed Krillin(smart move eh?).Thus Furiza got an ass beating the end.
Super Saiyan 2
Gohan and Gokou first reached this stage in the Hyperbolic Time Chamber. To bad you can only use it twice in a lifetime.
Super Saiyan 3
Gokou achieved this level when he was facing Majin Buu. Personally I think he achieved it before because it didn't take him very long to transform into it. Still Gokou was easily overpowered by Majin Buu.
*Note* Only Gokou and Gotenks have achieved this level. Others surpassed it.
Super Saiyan 4
I don't think this level is really a super saiyan because of its hair and the muscle mass doesn't really change. But in this level the saiyan is almost god-like. Gokou can use 10x Kame Hame Ha or Ultimate Kame Hame Ha. It's weird, in this level they get their tail back and have red-rimmed eyes(the red-rimmed eyes makes him look like he's high^_^.
Oozaru
Oozaru is that big over-sized gorilla that first appears when Gokou looks at the moon. Gokou was able to control Gohan when he was in the Oozaru stage.
*Note* Saiyans must have a tail to go to the Oozaru stage. If there tail is cut off they will grow another one, but if it is pulled out by the roots it will never grow back.
Ougon Oozaru:
This level was achieved by Gokou and Bebi Vegita in DBGT. This is the most powerful form a saiyan can achieve(ssj5 is bullshit). Before Planet Vegeta was called "Planet Plant" but of coarse they changed it to Planet Vegeta.