Oscillator not running
To me it appears the FOX OSC is not running correctly -- what would cause that with no loading?


Based on the updates, including info that those are deliberate PCB cuts near oscillator U10 and not solder splashes, and since you have updated that you were using a x10 probe with 15 pF loading, then my hypothesis would also be that the oscillator is indeed not working correctly and this is not a measurement artefact.

My one concern about the design is that C21, the decoupling capacitor for the oscillator, is not visible on the portion of the PCB layout supplied, and so it might be too far away. (Unfortunately the lack of annotations on the new photos mean that I still don't know which one is C21.)


  Can anyone make a suggestion on how I should proceed from here?


To confirm that the oscillator isn't working correctly, I would leave the existing one disconnected, and add a replacement oscillator upside-down (with adjacent decoupling capacitor) e.g. on the reverse side of the PCB near U7. Test its output to ensure it is as expected. Then connect the new oscillator's output to the PCB via the through-hole for the oscillator signal there.


  To me it appears the FOX OSC is not running correctly -- what would cause that with no loading?


Possible causes for the existing oscillator's apparent failure include overheating during soldering and ESD damage. Since you didn't see the assembly, you will likely never know the cause, unless you can interest the oscillator manufacturer in doing a failure analysis for you (which usually depends on your relationship with them e.g. future sales predictions, due to the cost for them in doing F.A.).


  Is there some issue that can occur during assembly that could be causing the issue? It is SMD with the pads under the OSC's can.


You explained that the output voltage is full-range i.e. 0 V to 3.3 V (albeit a sine wave) so I don't see an obvious sign of resistive loading. I mentioned other potential causes above.

Edit: A possible assembly issue are the added pads on that oscillator along the "long" sides. Those extra 2 pads are marked as "N.C." and also "TP" in the oscillator datasheet.


Source - datasheet page 6  

Those pads absolutely must not be connected to anything. Hidden solder bridges during assembly to those pads (terminals) underneath the oscillator may cause incorrect behaviour, yet would be undetectable when checking for correct connections just from the 4 "normal" oscillator pads.

In summary, after checking about those "N.C." oscillator pads, I would move forward by confirming failure of the existing oscillator by substitution. However, especially without you seeing the assembly (and device handling procedures) at the assembly house, I don't know how you will find true root cause, without an F.A. of the oscillator (when you have confirmed, by substitution, that it has indeed failed).


Newly added update to question:


  I acquired 10 prototypes. I've tested all 10 and the same OSC behavior.


That new info is the first time that anything more than one affected PCB has been mentioned. I will leave my answer in place as it shows conclusions which are still valid, but it makes some possible causes less likely, of course.