As posted at Oni Central, a release date for Oni has apparently been confirmed by ps2.ign.com in their listing of PS2 launch titles. From the launch list:

Listed below are titles that are sure to arrive after launch day and before year's end 2000:

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Oni (one week after launch) (Rockstar, Action-Fighting)
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The PS2 launches on October 26th, which gives us a seven day window for Oni's launch. Of course, nothing is official until Bungie says so, but this is the closest we have yet. And launching with the PS2 instead of a week later would go far to improve Oni's sales, so one might reasonably expect extra effort to go into meeting that deadline. More info as it comes...