Three sets of programs:
Set 1: Takes DOs and FMGs with somewhat regularity, maybe at least once every couple of years. They are less competitive and do not attract a ton of stellar US MDs. Doing average on your Step 1 will give you a reasonable shot at matching.
Set 2: Open to the possibility of a DO or FMG, has taken one or two historically. These can range from mid-tier to extremely competitive. Most of them expect to see a good step 1 score from all of their applicants and many will have step 1 floors (set at 220, 230, 240, etc) and regularly get plenty of applicants with very good scores. A stronger score, lets say >20 points above the typical for a program will turn heads. But, as it is for every applicant, then the other factors come in. And when it comes to those other factors, DOs and IMGs tend to suffer. Worse LOR, less research, etc. It isn't the DO that gets in the way, but the tendency for a lower quality applicant. FMGs tend not to suffer from this as much. They tend to have stronger LOR, research, etc. But they may have other issues (not for this thread).
Set 3: Never takes DOs, always has plenty of US MDs to fill their spots and chucks DO applications regardless of score. Reviewing applications and interviewing candidates is time and energy consuming. Every program employs some sort of filters. Some choose to simply not interview DOs because they know it won't hurt the quality of their class. They also tend to be the larger programs that already have a nightmare of time organizing their recruitment.
Is it discrimination to use grades or test scores to select applicants? At some point they are simply selection criteria. A fair argument can be made that DOs do not get the same education that MDs do, thus they are less qualified. Is it a gross over simplification? Of course. Is it something I ascribe to? Not particularly, but it is pretty hard to argue against, especially when a lot of the education has to be subjectively evaluated in the clinical years.