It has what many call the optical compressor smoothness and “chewiness” that many players describe. But it’s less than the Diamond and Union compressors, so I would say the Omnicomp is a warm smooth clean.
The Diamond is a thicker and darker version of that tone, maybe punchier too, and the eq controls are really useful and well-thought out. The LED compression meter changes colour which is nice too.
The Union sounds richer than the Diamond but less dark, less punchy, no eq, also has coloured LED.
I also have the Empress, Origin Effects Cali, and Fairfield Acountant compressors, which are FET compressors. A little harsher and more metallic than optical one might say. The Cali sounds big and darkish/growly, Empress is clean but has a tone switch to modify mid-frequency presence and introduce a very subtle sparkly overdrive. The Fairfield is not designed specifically for bass guitar but it’s mostly transparent, a little thin and metallic and dirty, a little bit of bass eq boost somewhere helps.