Kerry Wong fixed his HP 5350B OCXO. He documented the whole process on his blog:
Last time, I did a teardown of an HP 5350B microwave counter. And as I mentioned in my teardown, the oven-controlled crystal oscillator (OCXO) did not appear to be working as the OVEN indicator would not go off even after hours of use but the counter appeared fully functional otherwise. So I decided to disassemble the crystal oven and see if this problem can be fixed.
Find an HP 5350B microwave counter teardown here.