2) I don't know that would come out with something along the lines of "Oh
  my god!" It doesn't sound too japanese to me..

   It may not be something that any of the characters would literally say,
but it's inclusion is completely justified. Here's why:

   We write in English(1) for a mostly English speaking/reading audience.
As such, our medium is the English phrase and mode of speaking.

   "Saburo stared, aghast, and then doubled over and began to vomit. 
In the distance, sirens began to be heard. He prayed that they would 
get here soon, oh God, please let them hurry. The notion that he 
would have to go a different way from now on meandered into his 
thoughts."

   While there are Christians living in Japan, nothing in the Anime or
manga would indicate that either the Tendos or Saotomes are; in fact, the
sealing of Happosai suggests that they are Buddist. This would at first
seem to indicate that the use of the phrase "Oh my God" is one that would
never pass their lips. And technically, that's correct.
   However, I and most of the list have no idea of what the equivalent
phrase would be. In the passage above, Saburo isn't really begging God (at
least in the Judeo-Christian sense) to make the ambulances hurry; he's
roughly thinking, "I really, really hope, with a enormous amount of
feeling, that the ambulances and police will arrive soon." It's written as
"my God" because that's the phrase that most of us, even those who have no
particular religious beliefs, would use. And thus, we identify with the
thought, it becomes more moving for the reader and a clearer medium for
the writer.
   Imagine that you walk into a dirty room and remark, "God, what a mess."
Now, whick subsitute comes closer to the true meaning?
   "Boy, what a mess."
   "Our heavenly father, what a mess."
   The term "God", in western culture in general(2), can be used as an
expression of emphasis. Seeing as the language the fic is written in is
English, the use is perfectly acceptable. 
   

Ukyo: Ranchan!
Ranma: Ucchan!
Tsung: Now that's more like it.
Akane: (indignantly) What am I, chopped liver?

   I sincerely doubt that the Japanese use that last expression. However,
Akane WOULD have said something similar in tone and meaning, thus, we use
an English slang expression.
   Firthermore, playing on word and expressions, or witty (well,
humorous) repartee are a hallmark of many of the best fics. Obviously the
characters wouldn't use the exact phrase/joke/pun mentioned, but we can
assume that they use something eqivalent.
   Finally, think of just about any novel dealing with a alien or
otherworldly culture. Are English phrases and expressions used in it?
Probably. To throw in the occasional, "Burn me!" or "By the Light!" adds
color and flavor, much like the sprinkling of Japanese in fanfics does.
But the novel still uses English phrases and expressions.
   Now, if the author had made Ranma go to a cathedral or Synagogue to
meditate and pray, THAT would have been a mistake and a detriment to the
story.

(1) - I use the word "English" in this context to refer to the written
language used by most of the United States, Great Britain, Austraila, and
a few others.

(2) - English, German, and French all use the word for "God" in this
manner, and it's quite possible that other languages do too.

- Mike of No Particular Title or Handle
===/\=====+==================================================================
  /  \    l   Mike Loader - Lincolnite, Amberite, Illuminatus, Discordian,
 / () \   l      Journalist, Author, Traveler, Historian, Java Fiend.
/      \  l        Trier - LA - LV - Kiev - Reno - Hong Kong. Fnord.
--------  l                    mloader@scs.unr.edu
==========+==================================================================