Oregon Trail was a study conducted by the US Army Combat Development Command between 1963-1965 on "the use of nuclear weapons in land warfare." It involved, as one reference indicates, a "Field Test, 30 war games, 41 sub-studies." It is elsewhere credited with suggesting some of the positions that lead to the doctrine of "flexible response," which was put in contrast with the previous Eisenhower document of "massive retaliation," which basically involved what was colloquially called a "wargasm" in response to any instigation of "general war." "Flexible response" was about the possibility of avoiding total escalation into mutual nuclear use, through the use of tactical nuclear weapons both as a way to counteract Soviet conventional advantages, while at the same time giving the Soviets reason to end fighting early before going fully nuclear.

The longest description I have seen of Oregon Trail comes from a symposium from 1969 ("Proceedings of the Tactical Nuclear Weapons Symposium, September 3-5, 1969," Los Alamos Scientific Laboratory, LA-4350-MS"):

The Army in 1963 undertook a comprehensive and deliberate study, known as Oregon Trail, of the uses of nuclear weapons in land warfare. Oregon Trail was completed in February 1965 and recommended new organizations, new equipment, and a doctrine for the use of nuclear weapons in land warfare unlike previous ideas. This system of organizations, weapons, and doctrine was geared for attainment in the period 1968-1972. It relied heavily on battlefield dispersion of self-contained units. It emphasized the attrition nature of two-sided nuclear warfare. It stressed firepower, both nuclear and nonnuclear, and target acquisition, and tended to subordinate maneuver so long as the enemy retained a nuclear capability. It proposed great depth to the defensive position and did not seek to avoid enemy penetration of the spaces between dispersed units. Enemy elements, acquired in these spaces, were promptly taken under fire—either nuclear or nonnuclear. Great reliance was placed upon the 107 mm mortar, improved fragmentation munitions, a conceptual rocket delivery system, forward area air defense systems having passive acquisition means, and an effective anti-tactical missile and aircraft system.

The Department of the Army, in reviewing this study, determined that the revolutionary change in organization was not feasible by the time period 1968-1972 nor did it consider that the conceptual weapons upon which the concept relied were reasonably attainable in that period. On the other hand, the Department of the Army endorsed the concepts of widespread dispersion, great depth, a battle of attrition during two-sided nuclear conflicts, and postponement of decisive maneuver action until the enemy nuclear capability was substantially reduced. The Chief of Staff, Army, directed that these approved concepts be incorporated into a doctrine adaptable to the existing ROAD organization and equipment and that the doctrine be tested in the field.

Troop Test Frontier Shield was conducted in Europe in the winter of 1966-67. While its findings were not decisive, they indicated that the postulated doctrine was not workable with the equipment available and that a major command and control problem exists in implementing the conceptual ideas of Oregon Trail within the Frontier Shield postulated doctrine.

A later commentator in the symposium says: "One surprise of Oregon Trail is the apparent requirement for some 30,000 or so nuclear weapons." Which is rather interesting.

What exactly did Oregon Trail conclude, beyond this? That I do not know. What exactly were the tests? Ditto. I have not seen anyone extensively reference the content other than the vagaries provided here. It might be interesting. It might not be. It sounds like a long report. It's probably FOIA-able, but if it hasn't already been partially declassified it would take years to get results. (Maybe, if I get the time today, I'll file a request... you never know!) It would be interesting to compare it to other analyzes of this topic from different eras (like Project Vista, 1951, which looked in part at the role of nuclear weapons in the defense of Western Europe, and has been partially declassified).