San Juan Car Co. produces kits for a working type E coupler suitable for O and On3. As with all of their products, they are extremely detailed.
I have not spent much time testing them so I can not comment on their reliability but I have read some forum threads and on some Yahoo Group threads mixed reviews on just how reliable they are. I do know, that prototype modelers such as Gene Deimling of Proto48 Modeler uses them in most, if not all his models.
The couples are very durable molded plastic kits that assemble in just a matter of minutes. No glue is needed for the four parts provided as they are built like their prototypes. The casting have a lot of flashing so you will need to clean them up prior to finishing them. Adding to the realism, you can use a couple lift bar attached to the latch pin to uncouple your train.