Hi miden.
I have the 88 key edition myself so I will have to compare against my past experiences to explain the difference.

The Pa2X uses a FATAR key bed and the OASYS 76 uses a Yamaha bed, the exact same Yamaha bed that KORG use on their Triton Classic, Studio and Extreme keyboards. The Yamaha bed is very light and soft to the touch compared to the Pa2X which is a heavier weighted key. The feel of the key is different and with the yamaha bed there is glassy / polish finish to the keys where the FATAR has a dull finish with an almost grain like feel to the black keys. The Yamaha keys are also rounded more on the corners so when your playing chords spread out, the feel of the neighbouring keys tipping off the side of your finger tips is more pleasing.

They also produce a different sound when played. The FATAR has a nice soft sound almost like playing on keys backed by a fine thick felt stopper, where the Yamaha beds can have more of a thin felt meeting a metal stop sound to them when in their fully down position. The Yamaha beds can also get very noisy over their lifespan because of how the mechanism works in them and how the key is stopped when in it's fully down position.

That said, I sill personally prefer the Yamaha beds regardless of the noise they can make as they get older. The FATAR bed is nice to play too, but there's just something about it that feels “floppy” if you know what I mean.

Hope that helps a little.
James.