Op amp picks up power rail oscillations from boost conveter
Yup, opamps will pick up some of the signal on their power and interpet that as input.  This is exactly what the power supply rejection spec is there to quantify.  A ideal opamp would have inifinite power supply reject, but actual opamps have rather less than that.

Even with the power supply rejection spec, you can't take it at face value.  It is very rare that the freqency is specified, so assume it is at DC only.  The active circuit in the opamp that allows it to be relatively immune to power signals only works over some frequency range.  It may be less susceptible to power signals at high frequencies because the overall response of the opamp falls off with frequency, or it may be more susceptible because the active rejection circuit can't deal with the high frequencies as well.

Whenever you are amplifying small signals by a large gain, you have to filter the power supply.  This is standard practise.  Even a little power signal into your small signal can make a large mess downstream.  For sensitive amplifiers, such as when amplifying a microphone signal, always filter the power supply.

In your case, put a ferrite chip inductor in series with the power lead to the opamp, followed by a substantial ceramic cap to ground.  Something like a 1 &micro;H or so chip inductor followed by 10 &micro;F to ground is usually good enough.  This will form a L-C filter that strongly attenuates the high frequencies.  These chip inductors usually have a few 100 m&Omega; resistance, which forms a R-C filter with the capacitor that usually attenuates lower frequencies before the L-C kicks in.  For really sensitive circuits, put two of these filters in series on each power feed.