Is it ok for PhD to come from same school as BA? If my ultimate goal is to obtain a tenured teaching position, is it more desirable to get a PhD from a different university than the one where I obtained my BA? What are the possible pros and cons?

The original university has a large philosophy department, which is also a factor for me.
Many people refer to PhD programs accepting students from their BA programs as "academic incest." (The same term is used for hiring PhD students from the same program into the tenure track).  The reasons why doctoral programs generally don't want to commit academic incest are:


As with physical inbreeding, academic incest reduces the intellectual gene pool. You've already studied under a singular group of faculty for your BA, go to another program for your PhD to widen your intellectual horizons. 
As with physical inbreeding, if your program is known for academic incest, it dissuades possible future candidates from applying -- or at minimum, feeling that you prioritize your 'own' over others. 


There are clear reasons why a program might not want to be known for incestuous tendencies. However, the question is whether this hurts a candidate. Here, the risk/benefit analysis is a bit more murky.

Pros: If your program is indeed the best in X-Studies, why not remain at the best?

Cons: There is a risk that you might be seen as parochial -- that you only really know the intellectual currents at your single alma mater. To be honest, I'm not sure how significant a risk this is in terms of jobs although I would for the reasons of scholarly diversity above, still suggest that students not apply to their own PhD programs.


Fine print: Note that claims of the negative aspects of academic incest are much more pronounced (and pernicious) in the humanities and social sciences; and are much less common in the lab sciences. If there's only one university lab in the USA that does the type of gene splicing you want to do and you're continuing a project that you've done as an undergraduate, it'd be silly to think of going elsewhere.