The main factor for the increased casualties at Omaha was the weather. It led to a cascade of other problems. If they had better weather, all sorts of things would have happened differently.
The bombing of the shore installations would have probably gone off. Low cloud cover caused the bombers to miss their mark significantly. This kept the beach fortifications intact and allowed the Krauts to put their MGs on the cliffs firing down.
If the surf was not as high, they would have been able to land the amphibious Shermans that were almost all drowned. If I remember, only one got onto the beach, and it did a bang-up job taking out German emplacements. Twenty of them making it to the beach would have been devastating.
There are a multitude of these what-ifs, and they all go back to the marginal weather. Ike knew this going in. It was a crap shoot.
Read:
The Far Shore (Annotated)
by Edward Ellsberg (Author)
Ellsberg was an informed eyewitness at Omaha.
