High-speed IC GND next to oscillator GND
Given the oscillator IC will have finite PSRR, of course the highspeed IC will affect the oscillator IC. But how much affect (phase noise, timing jitter) will you tolerate?
Let us place some (worst case) numbers on the affect.
Assume 1 nanoHenry coupling. Assume 10mA/1nanoSecond from the highspeed IC thru its GROUND via.
The induced voltage will be (V = L * dI/dT) = 1e-9 Henry  * 0.01 amp/1nS ==

10 milliVolts upset in the Ground via of the oscillator IC.

Do you care?
If there are external Tuning voltages, you may care.
If the resonant currents of the oscillator are circulating EXTERNALLY, then you care.
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Now let us compute the induced time_error (time jitter, sort of like phase noise, but not random).
Assume 10 MHZ oscillator, with 1 volt Peak signal inside that oscillator.
The SlewRate is 2 * PI * 10MHz * 1 volt = 63,000,000 volts/second on the zero crossing.
And let us use that 0.01 volts as the &quot;noise&quot;, the upset of the zero crossing energy.
Tj = Vnoise/SlewRate = 0.01volts / (63,000,000 volts /second)
Tj = 1% of 16 nanoSeconds, or 160 picoSecods.
Is abrupt shifting of zero_crossing, of that oscillator, on the order of 160 picoSeconds, wherever the MCU dumps trash thru that Ground Via, a big deal to your system?