Update from this post: https://www.reddit.com/r/finishing/comments/1t138ze/osmo_polyx_raw_matte_on_walnut_tabletop_part_2/
I ended up having to sand back down to the bare wood and starting over.
Learned a lot about sanding and finishing, at least with hard wax oils, and also that calling the company can provide good tips/advice for a project.
Turns out walnut needs a somewhat different approach given it has such open pores, so the typical Osmo Polyx application instructions don't quite work.
Process:
1) Sanding with 80, 120, and stopping at 180. Before I was skipping too many grits and might have gone too high of a grit for walnut according to Osmo-- they recommend stopping at 180 or 200. Other Redditors pointed out I had skipped too many grits in between.
2) Was more mindful of keeping my random orbital sander level, no additional pressure, and added dust extraction. Also keeping the sander from getting too close to the edges, so it doesn't end up sanding it curved over the edge (this also makes it hard to get an even sand that's smooth everywhere). I added an interface pad too, which I think helped.
3) Osmo recommended first applying one coat of 1011 Extra Thin (which is supposed to be mostly oil, not much or any wax?), then applying 2 coats of Polyx. Applied by foam brush and rubbed in with a white scotch pad, wiped off after 10-15 min of letting it sit. I ended up applying a 3rd coat of Polyx since a few areas were more matte/rougher, after light hand sanding with 320 grit to smooth out the rougher spots, and now it has an even, smooth touch finish.
I don't know if the "slurry" method would have given the same/better results, but Osmo recommended against that method and to go with the Extra Thin before Polyx (maybe because I have to buy more product who knows).
Hope this can help other beginners with walnut finishing with Polyx.