The Allies pounded the crap out of all the invasion sites during the early morning of the 6th with heavy bombers. The strikes at Omaha were less effective than hoped (the plan was to have craters on the beach for the infantry to hide in, as well as dust up the fortifications) since the decision was made to bomb parallel to the shoreline to minimize the risk to the fleet. Unfortunately the bombers at Omaha were late due to English weather, were afraid of hitting the landing forces in their Higgins Boats, and dropped too far inland.
Napalm was not quite ready for Prime Time during Overlord (it made its debut six weeks later during the break-out from the hedgerows near St. Lo. It really wasn't until early 1945 that the Americans really figured out what they had on tap when the CAS units started incinerating IJA bunkers with the stuff and it became a tactical weapon of choice.
The Allies didn't spend a lot of time doing pre-invasion bombardment with the gun line because it was thought to be counter-productive. The idea was to make the landings a surprise and keep the Germans believing it was a feint until it was too late to bottle up the landing forces. Sending in a half dozen BB to blow the pougies out of everything was contra-indicated.
Also, the reality is that the Allied Intel units screwed up at Omaha. They thought that the troops holding the heights were low quality "combat rejects" with low morale and less than a full TOE who would break and run. Instead it was a well trained up and fully equipped Heer Division (the 352nd) with all nine battalions close to full manning and with virtually every bit of equipment the TOE provided.
Lastly, while Omaha was bloody as all hell, the American troops were off the beach within three hours of landing. The losses inflicted on the 1st and 29th Divisions, while heavy at nearly 3,000 K/W/M for the two divisions combined in the first week, were also not even the worst suffered by two American amphibious divisions during the month of June 1944, that dubious distinction going to the USMC 2nd (2,514) and 4th (3,628) Divisions on Saipan. As you can see, just the 4th Marine division took more severe losses that BOTH divisions that captured Omaha.
This does not alter the remarkable courage shown by the men of the Big Red One and Fighting 29th in any way, but it is a rather interesting comparison nonetheless.