Many years ago the late Ed Ravenscroft had a beautiful operating hump yard on his HO layout in Glencoe, IL. It got a big write-up in Model Railroader around 1961/62. Ed modeled the retarders on the tracks but they were non-working, at each one he used a puff of compressed air (triggered by a relay) to slow the cars as they rolled downgrade from the hump. You could use the same method in O gauge, or maybe use toothbrush heads (or something similar from the hardware store) to slow the cars by rubbing the bristles against the wheel flanges as they pass through your retarder.
The short answer is no, there are no commercially produced retarders in O; at least not functional retarders. There are several people who have modeled hump yards in O, both 3-rail and 2-rail, including forumite Bill DiMenna. I'm not sure what, if anything, Bill used for retarders. Most folks who have modeled a hump yard in O have done it without retarders because scale cars do not coast like the real thing. I have seen successful retarder systems in HO using both compressed air (nozzle blowing against the cars as they pass) and magnets (artificially "weight" a car with steel plate at is passes). All the systems I have seen in person were scratch built.
I've seen some old articles where cheap paint brushes with the bristles up through the ties would rub against the axles of a car to stop it or slow it. That could be adapted to O scale/O gauge fairly easily. Would require some good carpentry to cut the slots into the sub-roadbed. Also, weighting the cars to NMRA recommendations (greater than the recommendation for smaller cars like PS2 2-bays and ore cars) would pretty much be a requirement
Matt Jackson "The best service you can provide for the hobby is to pass on what you have learned."
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