A little history first.
Last year water pump went out. Took it to shop to get fixed (I didn't have time or desire to do it)
They told me I needed to have an oil cooler rebuild also.
A few months later I ordered the kit from Guzzles DieselOrings and rebuilt it. Everything went smooth no issues.

Ran it several days before doing coolant flush and all seemed to be well.

Well not anymore oil is getting in the coolant again.

Is the oil cooler the only source for this problem in the 7.3's or is there another possible source?

Thanks

 

There is another possible source, but there would be more fuel than oil in the coolant if that was the case. You likely nailed it - if no new oil is in the coolant.