In terms of timing, that is up to you. The education is far inferior especially with regards to physiology, biochemistry, molecular biology, pathology, microbiology, pharmacology, and systems. The philosophy no longer really plays a role in the educational process. You will spend hours in OMM (Osteopathic Medical Manipulation) courses and labs. There is no research or at least quality research to support any of their techniques. You are urged to "believe" as if it is was a religion. Furthermore, OMM will not help you if you are planning a career in path. You are not looked down on for taking the USMLEs. The schools do not encourage this, but you are at a major disadvantage if you want to apply to a competetive program in any field. The COMLEX is a poorly written exam, and the grading is not comparable to the USMLE. It makes it easier for the programs to compare apples to apples as opposed to apples and oranges. With current curriculum changes at my school, students will not be adequately prepared for the USMLE. It is not about working harder or doing outside reading. I was given outdated information, and I was not taught how to think appropriately about clinical situations. Most, not all, of the hospitals where DO students rotate are small community hospitals where you will not see the pathology you need to see as a student and the teaching is often lacking. The schools mostly hire their own graduates preventing an influx of new ideas and real curriculum changes. The line is "I wasn't taught that, and I did fine. Why do they need it?" The schools are behind the curve in teaching evidence based medicine and clinical problem solving skills. My school did not require me to read a single peer reviewed original article during my education. I have many many many more reasons. If I could go back, I would never go to a DO school. Depsite all that, I am graduating in the top of my class, and I am going to my first choice for residency. I am not just a disgruntled medical student.
I hope this helps.