First off, I'm another one of those 1911 biggots, but that doesn't prevent me from appreciating or accepting that other firearms can be just as good or better for certain situations or in certain hands. There are a few things that I consider for a SHTF gun other than the basic 'does it work for me issues': capacity, size, action, durability, and prevelance.
Concerning capacity: all of the guns that you mentioned will either have 13+1, 14+1 or 15+1, depending on magazine extensions.
Concerning size: all are of similiar size, and if they are going to be safe queens until a moment of need or for practice, I would suggest nothing less than a 5" barrel. The .45 ACP was designed for the 1911, which has a 5" barrel. You will get the better performance from the round if you stick with that length, or go longer if you want to or can.
Concerning durability: all have similiar tolerances. Taurus has been a little behind on that department, but if current tests are true, they are just as likely to consistantly work as the other two.
Concerning prevelance: this is what organizations use the pistol, and the aftermarket availability for standard and 'tweeking' parts. Glocks have this catergory hands down, currently.
Now on to the specific pistols. I have shot both the Glock 21 and the XD 45. The OSS is so new that none of my friends nor the local ranges have them for rent/use. the Glock that I shot had a 25 cent trigger job done to it, making the pounds of pull to around 3.5 lbs., which is similiar to my 1911. You might not want this since you are used to revolvers and the longer, stiffer trigger pull. I liked it, though, and I am planning on getting a Glock 21SF, doing the trigger job, and having it as my SHTF gun. Since local LE uses it as well, I will have plenty of opportunity to 'scavange' if the Zombpocalypse happens. The gun was extremely accurate for me at 40' (all in the 9 ring) and that sold me on it. Also, you can get a 2 round magazine extension, which brings the total rounds to 15+1, which is great. Remember, if you get a 21SF, only the new generation magazines will work in it.
My brother has an XD .45, which I put about 50 rounds through so far. It has a very different recoil than the Glock. It felt snappier, with the barrel rising a bit more than the Glock and my 1911, while the Glock felt like it was pushing back into me. I kept it in the 8 ring at 40'. Comparing the two, I would go for the Glock.
I know nothing about the OSS eccept what articles that I have read, and that it has a repeatable trigger. This is the only plus that I can see about it without having fired or handled it.
Of the three, if I had to buy one this second, I would choose the Glock 21SF.
Other guns to consider: Swith & Wesson makes their M&P in .45 now. A true American company, and the M&P has been accused of 'out Glocking the Glock' in durability and accuracy. The FN P45 is also a hi-cap .45 that you get in multiple configurations, one of which includes magazine capacity - 10+1, 14+1, or 15+1. I have held a few, and they are, to me, more ergonamic than any mentioned above. It comes with 3 magazines and is about the same price as the Glock. I am also considering purchasing one when funds allow.