Over time I've changed my advice on tankmates, due to both experience and learning more from reading.
I tried a pencil urchin (you can't choose any urchin with sharp spiines). This worked out well, but after the first year of being in the tank, the urchin was large and begain to seriously eat my coralline algae. Then I read that such an urchin needed a 200 gallon tank so as not to deplete the coralline algae.
Starfish are difficult to keep and don't seem to thrive in our tanks.
Brittlestars can be quite good, as long as you avoid the large green brittle star (serpent star).
Snails and small hermits are good - you octo may eat them when he's young, but later prefers other food.
Some people have tried peppermint shrimp and in certain cases the octo considers them tankmates and does not eat them. I sometimes put 25 small shore shrimp into the tank - one or two didn't get eaten. They lived for a long time, were cute and could be hand fed, and were wonderful scavengers.
Fish are not good. Clams will be eaten. Nudibranches, sea hares, sea slugs, etc. are to be avoided.
Mushrooms are good, but remember that an adult octo, especially a large one, may romp through the tank and then create a den - and nothing is sacred. However, mushrooms like out of the way places, so it's worth trying. As for featherdusters, octos have been known to snap off large featherdusters and stick them in front of their den entrances. Clumps of small featherdusters might survive.
Don't forget the value of brightly colored shells. If you use those, you might not need so many tank mates. Also, after experiencing den-building, I'd highly recommend having a bunch of lighter material like shells in the tank, otherwise your female octopus will be heaving 10 lb pieces of live rock about (and believe me, they can do it!)
Of course, like it or not, you'll probably have bristle worms!
Any one else have good experinces with other sorts of tankmates?