As with most cichlids, aggressiveness/territoriality is really a matter of individual personality/disposition.
In general O's are the pussycats of cichlids, pretty mild mannered and laid back...
However sometimes we run into a O who thinks like a RD.
1) Genetics, simple personality characteristics... Mild mannered, average, grouchy or psycho.
2) Age/maturity, often young adults can be very aggressive/territorial when reaching maturity and defining their territory.
Some seem to have a penchant for picking on bottom feeders at this stage.
Most do grow out of this and mellow out nicely.
3)Available habitat/territory(see 2), lack of sufficient habitat/territory due to overcrowding can lead to increased aggressiveness towards tankmates.