All the telegrams of support that Lt. Col. Oliver L. North held up for the television cameras at last week's Iran-contra hearings were sent with a little help from Western Union Corp.

Soon after North began his testimony on Capitol Hill, the communications firm slashed the cost of sending a wire to the former White House aide from the standard $10.75 to a $5.95 special.

The cheaper "Opinion-Gram" is generally available only for telegrams sent to the president and members of Congress.

Last Sunday, Western Union began to promote the "Ollie-grams" in small classified advertisements on the front page of The New York Times. "Make Your Voice Heard at the Iran-Contra Hearings. Call Western Union," the ads said.

Western Union spokesman Don Dutcher said the firm was trying "to be responsive to a very heavy public demand that we extend our service to include committee witnesses."

He acknowledged that the cut rate did not apply to Iran-contra witnesses who preceded North before the House and Senate select committees, but said it has remained for the rest of the hearings.

Dutcher said Western Union is barred by law from taking political positions and that it was merely trying "to acquaint people with the service." He said North was free to capitalize on the thousands of telegrams "because they were his property."