Part "O" is Eureka's famous "Groom-a-Pet" tool to clean the fur of dogs and cats! It seems to have appeared with the plastic tool caddy that came with the Empress II canisters and some uprights of the early 1970's.
All the brown attachments (F, R and V) that are designated for Singer vacuums are pretty much identical to Eureka attachments as they were made by Eureka for Singer in the 1960's. In the 1970's, Singer's attachments looked very similar to Eureka's, but they had more "squared" necks into which you inserted the wands or hose handles - for example, the floor brush labelled "B".