Looking for the rubber O ring that goes around the neck of distributor housing for a 1951 car, 235 engine. None of the auto parts stores know what I'm talking about, thanks.

That is because there is no such thing. If there is a grove it is for oil return. The distributor must be free to turn in the block .

How do you keep oil from leaking around distributor neck. And I've seen pictures with an O ring in the groove.

Gene is correct.

No significant amount of oil will leak out around the distributor without an O-ring.
And WITH an O-ring the vacuum advance can't freely turn the distributor as needed.

If you have seen a picture WITH an O-ring, someone didn't know what they were doing. :-)