These two cars are great HO cars to convert to OO.

First we have the Mantua caboose. From the side this car is OO gauge, clearly over scale for HO. The only problem area is the width of the car which is not that noticeable in service. I converted this car with the original trucks, what I refer to as the Morlok method, as Ed Morlok described this in an article in The OO Road. The couplers used are the underset shank version of the Kadee, so that they would be higher without the body being too high over the trucks.

The other car is a TYCO flat car. This car is over width for HO and again clearly over scale. For this the conversion is also very easy, it just needs new trucks and couplers. Again I used the modified HO trucks on this car, mounted in the off center holes to get them to center out better. 

One other note, a very interesting resource on TYCO HO may be found at the TYCO Trains Resource. The car I used for this conversion is here.