The ornamental design for a portion of an oscillator, as shown and described.
FIG. 1 is a perspective view of a portion of an oscillator showing my new design;
FIG. 2 is right side elevational view showing the portion of the oscillator of FIG. 1;
FIG. 3 is left side elevational view showing the portion of the oscillator of FIG. 1;
FIG. 4 is top elevational view showing the portion of the oscillator of FIG. 1; and,
FIG. 5 is front elevational view showing the portion of the oscillator of FIG. 1.
The broken line showing of other portions of the oscillator in the drawings is included for illustrative purposes only and forms no part of the claimed design.