I own one, but I play 8 string guitar, so take my comments accordingly
It's a great OD. Has a nice "warm" character. Even at high gain settings it doesn't get too scooped or compressed.
I personally set the "first" channel to give me a little grit and warmth (mild OD), and the "second" (+) to be the heavy sound.
That's actually why I got it. I wanted a 2 in 1 kinda pedal.
It reacts incredibly well to the low end.
There is however something I'm not too fond of. Mostly that the top controls are very subtle.
-the Clean Blend gets dirty quite soon on the dial, and I can't quite hear the clean too well in the mix. (That being said the low end stays pretty massive).
-The Tone knob hardly does anything as it affects a pretty narrow band of midrange. I'm of the mindset that if you have controls on a pedal they should do something noticeable.
-I have to crank the Bite control to get the top end to start really cutting through. I even sent the pedal to Heiko to mod it more for an 8 string range, but honestly I can't hear much of a difference.
Once again, these are mostly MY concerns as I use the pedal on an 8 string guitar so my needs cross over between a bass and a guitar.
In essence I love what it does to the low end, but I'm not too fond of how my top end doesn't quite keep up. So for bass I would imagine it's pretty stellar.
Solid build. Pretty wide range of tones from mild OD to balls out aggressive, but still in a slightly "vintage" warm way (this ain't a B3K).
I also quickly A/B'd it with my trusty BB Preamp, and while the Okko has more character, the BB treats my high range in a way I enjoy more (while I wish it had a clean blend and a second channel
Hope that helped a little.