Subspace oscillation was a periodic variance in subspace. Subspace oscillations were observable by 24th century Federation sensors. They could cause minor gravimetric vibrations. A series of oscillations was called subspace harmonic.
Subspace oscillations were one of the effects observed in subspace disruptions. When Deep Space 9 was threatened by a nearby subspace rupture around stardate 46853, the station's perimeter sensors picked up subspace oscillations. These oscillations caused the station to vibrate. (DS9: "If Wishes Were Horses")