Recently did a key bushing replacement and calibration and must have knocked something loose… For both Bass Synth and Bass String voices, the lowest octave is fine but he next octave plays the same note for every key. Upper Synth and String voices play at correct pitch across the board so assuming it may be one of the voltage divider ICs? Any help on where to look appreciated.
Since you said you did a bushing replacement it could be the J-wire for the highest bass note that plays was bent and is always making contact with the bus bar. If it isn’t that then it’s most likely a problem in the bass low-note priority circuit. I worked on an Arp Omni recently that had the same issue. It ended up being one of the NOR gate ICs on the lower voicing board. IC, 4025 triple 3-input NOR gate - Syntaur
If you haven’t replaced the tantalum capacitors in your Omni with new electrolytic capacitors those can wreak all sorts of havoc too. Also there is also a power supply modification issued by Arp that is highly recommended. It saves the ICs from a transient voltage spike that occurs with the factory Omni power supply design.
The J-wire for the highest bass note and potential problems in the bass low-note priority circuit are common culprits for such issues.