An oscillator was a type of electronic circuit.
In a holoprogram set in 1944 France, Katrine ordered Mademoiselle de Neuf to visit a man named Goulot in a cottage outside Sainte Claire. He had a wine cellar full of vacuum tubes, including high-frequency oscillators. Katrine was willing to pay between 300 and 500 Francs to buy such an oscillator, using it to increase the range of her radio. (VOY: "The Killing Game")
In 2370, when Miles O'Brien and Julian Bashir were trapped on T'Lani III, the two tried to repair an old communications device to call for help. Bashir was able to retune the device's oscillator, though the frequency still fluctuated. (DS9: "Armageddon Game")