The American oil company that owns The Observer denied today that the naming of a new editor-in-chief would affect the "continuing role" Douglass Cater with the British Sunday newspaper.

Cater, a former journalist and aide to President Lyndon B. Johnson, "has done an outstanding job as our representative in London," said Thornton Bradshaw, president of Atlantic Richfield Co. Cater "is wholeheartedly in support of the recent appointment of Dr. Conor Cruise O'Brien as editor-in-chief."

"Mr. Cater will continue to serve as vice-chairman of The Observer, and as president of Observer International Inc., which holds a controlling interest in the newspaper," the Bradshaw statement said.

The Washington Post reported on Sunday from London that pressures from the paper's staff to ease out Cater had culminated in the O'Brien appointment.

According to Bradshaw's statement, Mr. Cater was a principal architect in new position with the paper." O'Brien is a distinguished Irish diplomat, critic, writer and politician.

Reached in Jamaica, where he is on vacation, Cater said he had no intention of leaving the paper. "My role awas never designed to be an editorial one," he said.

Cater added that he regarded O'Brien's selection as "a triumph" because The Observer had simultaneously hired a man of "brilliance" and retained its editor, Donald Trelford.

Cater said he had met O'Brien before the Irish writer was selected, but "Mr. Anderson of course made the decision." Robert O. Anderson is the chairman of the paper's parent oil company.

"I love my role in London," Cater said, "and plan to continue it. I look forward to Dr. O'Brien's arrival as editor-in-chief."

Senior executives at the newspaper had contended that Cater had exercised a limited influence over the paper in the past and it would now be further diminished.

The Post had also reported that Cater's health was a factor in them ove. Tonight, he said he had been planning "slightly defective value" with which he has been living since a bout of rheumatic fever at 13. Cater, 54, said this condition "does nnot contribute to bad health" and "I lead an active social and business life."