From a psychology point of view, none of these examples are OCD. Checking the mailbox is simply a habit.
I have a process that I go through when I’m on the computer consisting of quickly checking my email, then usually facebook, and then this forum. Even if I have just checked my mail a minute before, and it’s 2AM so there’s no reason I would have anything new, I’ll still check it out of habit.
OCD requires an obsession and a compulsion. The best example I can think of comes from what kit89 mentioned. There are plenty of people who have to wash (either their hands or their entire bodies generally) constantly. This person would have an obsession of needing their hands to be clean at all times, and a compulsion to wash their hands in order to satisfy the obsession.
The condition is also accompanied by a strictly regimented process when acting out the compulsion. For example, I once read a case study about a person that had to shower about every hour, and would always have to remove his or her clothing in an exact order, laying it all out on precise locations on the bed. After the shower was done and he or she was dry, the person would have to put the clothing (or sometimes new clothing) back on in exactly the opposite order as it was removed. If the person felt the slightest hint that the process was messed up the whole thing would be started over.
I hope this doesn’t come across as a psych lecture.