People that can only see one thing or in most cases lacking vision completely.
A "onevision" therefore steal visions from others in most cases.
A "onevision" can also be called a "novision".
Picture of a "onevision" character:
Meaning: what a stiff vision lacking piece of shit moron,
One of those vision lacking thief's.
"O no, another onevision!"
Meaning: someone stole our invention, again...
But if you go deeper in the lyrics, you face some parallels with politics and New World Order theory.
One world, one vision!