Definition of "oftener"

A street in Constantinople is a picture which one ought to see once—not oftener.

1869, Mark Twain [pseudonym; Samuel Langhorne Clemens], chapter XXXIII, in The Innocents Abroad, or The New Pilgrims’ Progress; […], Hartford, Conn.: American Publishing Company. […], page 359

There were more sheebeens in Inverara than churches, which of course was natural enough, for alcohol cannot be put into a condensed state like the Grace of God, to be consumed on or off the premises. It takes up more space and has to be consumed in greater bulk and oftener in order to produce the desired effect.

1923, Liam O'Flaherty, “Tuesday”, in Thy Neighbour's Wife, 1st paperback edition, Wolfhound Press, page 127

The heavy duty carburetor oil bath cleaner oil should be changed and the oil reservoir cleaned every 5,000 miles or oftener if driving conditions warrant.

1954, Packard, 1955 Clipper Owner's Manual, page 29