That grey-haired guy in the back, that is you?
So everything looks alike in my lab, 5335A, 3325B, Rb standard.. and a lot other time-nuts stuff.
Well, the OCXO seems to be adjusted very well, I assume, you did NOT attach the Rb also to the 5335As REF IN?
1E-9 is a reasonable precision for the OCXO.
If you want to get deeper into stability figures, I really recommend to try the Time-Lab, it's really fun to dig into Allans Deviations.
An OCXO needs 48h to stabilize to a stable frequency, i.e. < 1E-9. So it should be powered on all the time, as it's the case of our 5335As. Consumes a few Watts, which I can afford, even here in Germany.
Rb clocks may need up to 5 days, to stabilize to about parts in 1E-11, so it's not recommended to switch it on for a quick measurement only.
You may also let it run, and feed to REF IN of your 5335A.
That would be the most stable oscillator for this counter.
P.S.: The back plane cooling of the Rb standard is definitely necessary.
Please trust an experienced time-nuts, or read the manuals of the LPR-101, or the EFRATOM FRS-K.
These specify the use of a metal plate, to limit the temperature.
In the LPRO manual, the MTBF figures go down drastically with rising temperature, and in the FRS-K manual, 65°C back plane temperature is defined as absolute maximum.
The 2nd effect after the limitation of the life time, is the stability of the internal ovens, which might be affected, or unregulated, if improper cooling lets the internal temperature rise above the 70°C of the first oven in the physics package.
Frank