Their motivations were certainly different, but they each struck at the heart of both the American people and the American government. Their methodologies were different, but the result was virtually the same--death, destruction, fear, paranoia, as well as patriotism, helpfulness, bravery, and virtue. They surely predicted and reveled in the first set of consequences; I'm wondering if they also anticipated the outpouring of America's best in the face of a crisis. I'd guess not.
The Code of Federal Regulations defines terrorism as 'the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in further of political or social objectives' (28 C.F.R. section 0.85)
Those that would commit terrorists acts predicate their potential power based upon the fear they can inflict upon others. Yes, American born Timothy McVeigh committed a terrorist act on the United States the same as those on September 11, 2001. The fact is both embody the Federal definition of a terrorist attack.
I don't see a lot of similarities between these two attacks. 9/11 was religiously motivated, while Oklahoma City was an idea driven by discontent with the domestic government. All of the hijackers on 9/11 were foreigners, and part of a larger organization of terrorists, while the Oklahoma City bombing was planned and carried out by just a few Americans acting alone.
Yes, both attacks were carried out against symbols of the US government, the World Trade Center as a symbol and heart of our economy, and the Oklahoma City building because it was the office of the Federal government, and large numbers of innocent civilians were killed in both, but as I see it, that's where the similarities end.
As the previous poster pointed out, both attacks were designed to take as many lives as possible, but they were also planned to have the greatest impact visually and in terms of attacking something that many people considered secure or somehow protected. Very few people could have imagined something like the attack on September 11 and very few people could think that McVeigh could successfully destroy a huge icon like the Murrah Federal Building in Oklahoma City.
So both of them accomplished in some ways similar things by demonstrating that any number of secure or protected things can be destroyed by someone willing to sacrifice their life or basically act so irrationally as to do something like this.
I certainly think that one can draw parallels between both attacks. The most elemental would be that they took place on American soil, something that the nation has not had to experience much of in its history. Both of them also were designed to take place in densely populated areas. The Murrah Building and the multiple sites of the September 11 attacks were not isolated from population centers, as both were designed to take a great deal of human life. Both attacks were statements being made against the American Government. While the Oklahoma City Bombing was driven by domestic discord and the attacks of September 11 by international antagonism, both were designed to be deliberate statements against the U.S. Government as well as its policies. On another level, both attacks transformed how law enforcement pursues terrorist. McVeigh's usage of fertilizer and parking the van in front of the Murrah building and leaving it unattended helped law enforcement to strictly monitor any item in a public setting that was unattended. The attacks of September 11 drastically changed how airport security screens passengers and the removal of sharp objects such as blades and box cutters is one of its legacies. Finally, both attacks have firmly imprinted the reality that terrorism is something with which Americans must live and the belief that "it could never happen here" forever disappeared from American consciousness.