It would be unconscionable to allow former house speaker Tip O'Neill {''The Speaker Speaks Out on the President,'' Outlook, Sept. 13} to escape unchallenged on his wholly inaccurate comments concerning the liberation of Grenada. Especially his statement that no Americans were in danger, and that the liberation had no larger significance than allowing the country ". . . to forget about the tragedy in Beirut."

It is a matter of historical record that the self-proclaimed Communist government of Prime Minister Maurice Bishop had considered using the American students at St. Georges University as hostages against the United States.

Grenada's ambassador to the United States, Kenrick Radix, addressed a Grenadian rally outside the United Nations on Nov. 15, 1979, and declared that the taking of American hostages in Tehran was justified in light of the shah's admission into the United States, and that Grenada would support the taking of U.S. hostages in that country -- meaning medical students -- to force the United States to "return" the deposed Eric Gairy. Later, the labor minister of Grenada, Selwyn Strachan, endorsed the idea in public speeches.

Internal documents captured from the files of the Bishop government clearly demonstrate that the medical school was closely watched and monitored. As late as April 1980 there were "assessments" presented to the Central Committee on school activities. When, in October of 1983, the various Communist factions turned to blood-letting, the American government requested evacuation for the students. That request was denied. The airport was closed, and a shoot-on-sight curfew was issued. The American government was aware, even if Mr. O'Neill was not, that U.S. citizens had been, and were, at risk.

Mr. O'Neill states that his greatest disappointment with President Reagan is that ". . . he didn't grow in office." My greatest disappointment with Mr. O'Neill is that after a lifetime of watching totalitarians march through history, he still has no comprehension of them, or their threat.

MIKE GIERE Falls Church

While I am willing to let history judge whether the policies of President Reagan or those of former speaker Tip O'Neill will be seen as beneficial to the nation and the world, I think it might be useful to place at least one of Tip's opinions in context.

My longtime political sparring partner writes, ''. . . today I feel even more strongly that we should not have invaded Grenada. Despite what the administration claimed, the students were never in danger . . .''

Given his busy schedule of dictating his memoirs and lecturing across the nation, my old friend may have forgotten the facts of the matter. Immediately after the United States landed on Grenada, the speaker asked then-majority whip Tom Foley and me to head a bipartisan fact-finding congressional delegation to go to Grenada and see what happened. When we came back, we reported to the speaker, and on Nov. 11, 1983, The Boston Globe carried a story headlined:

"O'Neill: Grenada Invasion Justified."

The story began:

''House Speaker Thomas P. O'Neill Jr., who last week expressed skepticism about the administration's invasion of Grenada, said yesterday the action was justified given the 'potentially life-threatening' situation for Americans on the Caribbean island.

'' 'The overwhelming consensus of the members of the delegation was that a real potential threat to the American citizens existed in Grenada,' '' said O'Neill.

On the same day, a New York Times story was headlined:

"O'Neill Now Calls Grenada Invasion 'Justified' Action."

The first paragraph stated:

''Speaker Thomas P. O'Neill Jr., who criticized President Reagan for 'gunboat diplomacy' in the invasion of Grenada, said today that a House fact-finding mission had convinced him the action was 'justified' to rescue endangered Americans.''

I sincerely hope that the rest of his book doesn't reflect such memory lapses. Perhaps Tip could borrow some of President Reagan's 3x5 cards.

ROBERT H. MICHEL U.S. Representative (R-Ill.) Republican Leader Washington