I think its safe to say that a realisitc D-Day mission is impossible with this version of OFP. Unless you ran delete scripts to delete the dead bodies, scaled down the accuracy of the Germans franticly firing as fast as they can, and scaled down the ability of allied troops to take out gun positions so easily, and then scripted in 'waves' of troops so that there won't be thousands of units in game at the same time....then maybe, give me a call if you want to try that out. I would love to help you out with something like that.
Someone mentioned using other allied forces, other than the American invasion. While any other allied invasion would be almost exactly the same as the American one, it would make more sense to perhaps do the more behind the scenes. Lets take an example of the tragic Battle of Dieppe by the Canadians. While the Canadians feigned an invasion at Dieppe British Commandos and Canadian Recon units went ashore to destroy some German facilitys and to steal some goodies down the beach from the main invasion force. While that Battle would be WAY too big to remake, the commando mission behind the scenes would be well suited for OFP.
I'm very confused by your post, because not only did I just give several examples of how to do this, I also mentioned it would be a mistake to try to emulate any actual mission based strictly on it's historical outcome. Read my Ranger assault on Pointe-du-Hoc example again. Now this wouldn't be a mission where you start in a Higgins boat, land, then scale the cliff with the rest of the battalion. In my example, I use a similar, but not identical, situation, by a smaller group of Rangers, a fictional mission, in which they
support the action the main body of Rangers are engaged in, they do not actually participate in it. They already start ashore, and can HEAR the battle, but not actually see it (Use weather to your advantage! Perhaps it is raining in "your" D-day mission, or the fog has rolled in, or whatever. The coast has quickly changing weather)
Similarly, the concept of the 506th and 508th PIR drops are a great idea for a D-day scenario. D-day was not just the events depicted in the movies. Although some of them were quite beleivable, they are not the whole story.
The attitude that rigidly binds your attention to
just the beach landings, or just the beach landings that were depicted in film (some of the landings were not strongly opposed), or to trying to 're-enact' historical events as they happened is the wrong path the explore. use your imagination. i have already given an example of a mission that will give the feeling of one of the famous events of D-day, without the need for hundreds of troops, and without pre-determining the outcome of the fight. there are many others. Do not be bound by strict adhereance to historical fact. Try a twist on some actual events leading up to D-day. For example:
Several missions including only one or two men were undertaken preceeding the invasion, to take ground level photographs of defenses, and to take soil samples to judge the suitability of the beach to support the weight of armored units. Take that event, and your example of Dieppe. Now put a twist on it, like this-
A five man commando team swims ashore on a beachhead
other than that of the one on the Cotentin penninsula in Normandy. Say, the Pas-de-Calais. It is D-Day minus one, the day before the invasion. They are to probe enemy defenses, blow up mine fields and destroy armored units and guns if possible, to convince the Germans that they are a pathfinder unit paving the way for the invasion of France, at the Pas-de-Calais.
Use your imagination and do not be limited to large scale landings including dozens of MG emplacements, hundreds of attackers, and a thousand or more mines. The airborne units had small skirmishes (if you will not believe me, read an account of their D-Day actions for yourself) and smaller missions can be adapted from primary missions, as in my ranger example, or a complete fantasy mission can be made, as in my 5 man commando team example.