Did Jackie O or didn't Jackie O purchase nearly 400 prime Martha's Vineyard acres? The most persistent rumor on that rumor-prone vacation island is that Jacqueline Kennedy Onassis did, in a deal surrounded by more than usual secrecy.

Feeding the rumor are these facts:

Last winter it was disclosed that Alexander D. Forger, one of the former First Lady's New York attorneys, had purchased for $1.15 million some 375 acres of unspoiled land belonging to the Hornblower family of the Hornblower-Weeks and Hamphill stock brokerage firm.

When asked if he had purchased it for Mrs. Onassis, Forger said yesterday, "I will neither confirm or deny. All I will say is that I am holding it in trust." (The name of the beneficiary does not appear on the transaction.)

Mrs. Onassis, vacationing at the Kennedy compound in Hyannis Port, could not be reached for comment.

A member of a preservation group which had vied for the property said with a chuckle, "We are absolutely certain it is Jackie - but we do not hate the proof. Unless, of course, you count the vary guardedness of the transaction. All you get is a cover-up from everyone involved. It is easy enough to say no if it isn't true," said Nicholas Freydberg.

The land stretches for nearly a mile along the ocean front near the Vine-yard's most sparsely settled village, Gay Head, and is described as "very romantic, very rare." Freyberg said, "We wanted to combine conservation - keeping about 60 percent of the land untouched - with some 12 sites for private homes." Members of his group, which included former secretary of defense Robert McNamara, would not consent to a purchase until the title to the land had been cleared. "There was some question as to title," said Freydberg.

Meanwhile, Forger was also negotiating with a Hornblower representative. "We think it was actually purchased last summer, but we first learned of it in January," said Freydberg. "We had been continuing to negotiate in good faith."

"If Jackie is the owner, we are satisfied the land will not be abused," said Freydberg. "I can't see her in the real estate business." The speculation is that she could turn it into a compound for her own family, similar to the Kennedy compound at Ryannis, 30 miles away away on the Cape.

The sale included the stipulation that zoning and land use rules of the state and local governments would not be violated. This apparently was an attempt to ensure that the 375 acres not be subdivided.

The Vineyard Gazette, which first reported the land sale, has taken a lofty view of the latest rumors of the purchaser. "The celebrity nature of the transaction is of no concern, to us," sniffed one executive of the paper.

Ironically, if a bill introduced into Congress three years ago had passed, it could have possibly thwarted the Martha's Vineyard purchase. The bill was aimed at curbing development on the Nantucket Islands. It was passed by the Senate in 1975, but died at the end of the session without House action. The senator who introduced the bill was Edward M. Kennedy.