Ozark Air Lines yesterday proposed flying between Dulles International Airport and St. Louis -- a route no carrier currently serves -- at fares 20 to 50 per cent the air fare from National Airport.

In an application filed yesterday with the Civil Aeronautics Board, Ozark proposed three flights in each direction each day using single-class seating on 100-passenger DC-9-30 jet aircraft. The "normal" fare would be $62.04 one way, a 20 per cent reduction from the current fare of $77.78 from St. Louis to Washington National. Ozark would reduce the fare 50 per cent, to $38.89, on the planned 9 p.m. departures in each direction on Monday, Tuesday, Wednesday, and Saturdays.

Ozark would also apply discounts of about 20 per cent to the flights that originate in cities beyond St. Louis but stop there before coming here.

In announcing the CAB application here, Ozark president Edward J. Crane said he was "optimistic" about getting approval for the new route because of the board's "recent activity" in encouraging low-fare proposals, the fact that no carrier serves Dulles from St. Louis now, and because Trans World Airlines has a monopoly route from St. Louis to Washington --it serves National -- since Eastern Air Lines dropped its service to National last month. (A spokesman for Eastern said yesterday the airline may resume its one-flight-a-day schedule next month.)

Ozark is a fast-growing "regional" carrier whose region's outside boundaries are Denver on the west, Minneapolis/St. Paul at its most northern point, New York in the east, and Dallad/Ft. Worth at its most southern point.

From Washington, Ozark currently has nonstop service only to and from New York City and Champaign-Urbana, Ill.

Crane said Ozark hoped the lower fares would be an "incentive" to passengers to use the more distant Dulles Airport. Both Eastern and TWA have the authority to serve the St. LouisDulles route, but haven't used it. Ozark's application is supported by the State of Virginia.

The CAB rejected Braniff Airways' proposal to institute a 50 per cent fare reduction "Home Free" plan on certain routes because it said it was troubled by certain cancellation and no-transfer conditions. The agency indicated it would consider a new proposal that lessened or removed the restrictions. Under the Braniff plan, a person cancelling a reservation within 14 days of the flight would forfeit the entire ticket price; transfers to other persons also wouldn't be allowed.

American Airlines yesterday said it would ask the CAB for permission to extend its discount "Super Saver" fares in 44 additional markets beginning Feb. 24, more than doubling the size of the program. One Washington route -- to Palm Springs, Calif. -- would be affected.