Historically, these plates were used on buses, taxis, livery vehicles and rental cars.
Starting in the 1970's, when unique livety-type plates were issued, OMNIBUS plates were only used on buses and taxis.
1960 BASEOWA-100 - OX?-999
The initial issue (OWA through OX?) has tab slots, but tabs were not used - the plates were validated with stickers.
Tab slots stopped somewhere in the early OX series, maybe corresponding with the start of reflectorization.
The tab slots extended into the OX- series.
Not sure where the cutoff is, but maybe OXC-100 or higher.
1969 BASE - REFLECTORIZEDOX?-100 - OXO-999
Some of the early OX series plates were reflectorized - from at least OXG to OXO.
STICKERED TO 1986Dave Zullo
1974 BASEOXP-100 - OXZ-999
The OXP series (and higher) were not reflectorized.
1979 BASEOYA-100 - OZZ-999(?)
The highest number known on the blue base is OZR-485.