Let me try to clarify some of the informations
To practice in California - you will need a CA medical license, and a job in California. DOs are licensed by the Osteopathic Medical Board of California (and not the Medical Board of California which licensed MDs)
Getting a job in California will be easier if you do a residency in California but that;s because it is easier to do a job search locally, but it is not required to do residency in California to work in California. Just like it is easier to get an engineering job in California if you attend a CA school than if you attend a school in Illinois - locals have advantages. How easy/difficult to get a job in California depends on your specialty, the degree of need (oversaturation vs severe shortage) as well as what unique skillsets you bring to the market. If you are in a field where it is supersaturated, then it may be hard to find a job in CA. If you are in a field where there is a severe shortage, you may have many competing offers. There are some areas of California where almost all fields are supersaturated due to location and desirability. Now if you are superspecialized, like a pediatric neurointerventional radiologist, and a place like Stanford or UCSF has an opening for a pediatric neurointerventional radiologist, then you're in luck. If they are not looking to hire, then you're out of luck (even if you train there). *plus academic medicine is a slightly different game than the private market but outside the scope of this post
Residencies in CA are more competitive in general because a lot of people want to either stay in California for residency or come to California for residency. In addition, there are a few high caliber university programs (Stanford, UCSF, UCLA, UCSD) that are extremely competitive, even for MD students. Doing a visiting student rotation as a 4th year student in one of the programs in California can help (if you excel), but can also hurt (if you are just average, or worse, are below average and don't know it).
To get an UNRESTRICTED license in California as a DO, you need to pass all 3 steps of COMLEX. California will not recognize USMLE scores for purposes of licensure for DO. However, you may need to take USMLE (step 1 and/or step 2) to be competitive for ACGME university programs. California does not require an osteopathic approved internship - only 4 states do (Pennsylvania, Michigan, Oklahoma, Florida). That can be done either through an AOA residency, a TRI followed by regular ACGME residency, or ACGME residency where the first year (PGY1) is approved by the AOA via Resolution 42 (please do a search on SDN as it has been discussed multiple times in the past). For DOs, California requires completion of 1 year of residency, whether it is ACGME or AOA.
Cardiology is a competitive fellowship that you get after completion of Internal Medicine residency (or pediatric cardiology after pediatric residency). There are both ACGME Cardiology fellowships and AOA Cardiology fellowships. In the near future, both will merge and will all be ACGME cardiology fellowships. As it stands right now, AOA fellowships are available only to DOs, either those who did an AOA IM residency, or those who did an ACGME IM residency, petition the AOA to accept that training as AOA equivalent via Res 56, then do the AOA cards fellowship. ACGME cards fellowship is open to those who did a ACGME IM residency, or to a very understanding program director who will take an AOA IM graduate with the understanding that the training will not lead to ABIM Cardiology board certification (will have to petition the AOA to accept the acgme cardiology fellowship as AOA-equivalent and take the AOBIM Cardiology boards). This will be moot when the AOA and ACGME merges.
The boards mean two different things. For medical students, "the boards" is the medical licensing exam, either COMLEX or USMLE. For practicing physicians, the "boards" or "board certification" means being certified by one of the boards under the umbrella of the ABMS (American Board of Medical Specialties) or AOA Specialty Certifying Boards. You do not need board certification to practice medicine in the US. It may be required to obtain hospital privileges, to accept government administered insurances (medicare, medicaid), to work for government hospital (VA, civilian contractors at military hospitals), or private insurances. It may also be difficult obtaining malpractice insurance. But there is nothing stopping you from opening your own practice and seeing patients. Whether that's a good idea ... that's a different story.
Hope this helps.
Clarification of Terms (for those new to the game)
ACGME residency - (Accreditation Council for Graduate Medical Education) usually on SDN is referred as MD residencies.
AOA residency - Postgraduate residencies accredited by the AOA instead of ACGME
TRI - traditional rotating internship, what old school doctors (both DOs and MDs) used to do after graduating from medical school and going into praactice (before residency was mandatory). Now available mostly in the DO worlds for those interested in an advanced residency that requires an internship before starting (ie Dermatology, Radiology, some Anesthesiology programs, Ophthalmology, etc) or for those who want extra time before starting residency.
USMLE - United States Medical Licensing Examination, what MD students have to take to get a medical license. Also the standardized exam used by residency and fellowship directors. Hence DOs taking USMLE since it is more familiar to program directors.
COMLEX - Comprehensive Osteopathic Medical Licensing Examination - what DO students have to take to get a medical license. Also the standardized exam used by osteopathic residency and fellowship directors. Required to take Part 1 and Part 2 (and pass) to graduate from DO schools.
Board Certification - being certified by a recognized board, either an ABMS board or AOA board. Board-certification, in its common usage, does not mean passing USMLE or COMLEX.
ABIM - American Board of Internal Medicine
AOBIM - American Osteopathic Board of Internal Medicine
