Keeping output of a phase shift oscillator in-phase with another sinusoidal signal
You need a device which is able to measure the phase difference (phase stector PD) and another device which uses this information for slightly changing/adopting the phase/frequency of the oscillator. In principle, this is nothing else than a phase-locked loop system. For tuning the phase shift oscillator it should be sufficient to vary one 45-deg element only (in case the variation is small enough and you have a working amplitude control).