OAX1 is an organ with arranger features and is exactly the same as all the other organs in the range.
The big 3 are dedicated arrangers that require plenty of workarounds if you want to add pedals and 2nd keyboard to operate like an organ. (An organ OS is way more flexible for using multi-keyboards and pedals)
With OAX you are also not limited to a limited number of sound layers for the right/left hand, as you have 16 sound layers which can be assigned to any manual you like. (All sounds have their own settings including key range)
OAX is a software-based instrument, so new features and sounds etc. can be added without needing to change the instrument. (OAX is over 10yrs old and early models can be upgraded to the latest versions via software, (Pretty much everyone that owns an OAX instrument has already done that over time) which up until V3.00 have been completely free)
The Real Drums (Audio drum loops) use standard REX files, so you can edit and change them in an external DAW.
OAX also has a VST Host built in, so you can use pro sound and effect VSTs as if they were always part of the instrument.
The downside is that as the organ world is a niche market (The arranger market is also Fastly approaching niche) manufactures have to charge more to make a profit.

Bill