Sub Articulo 88.1

That sounds like a standard limitation of bibtex.  Generally there
seems to be two ways it can treat titles, the first is to convert them
to lower case, the second to leave them as entered.  The trouble with
the first approach is that it leads to situations such as an element
symbols such as `Yb' being converted to `yb', unless you know of the
problem and enter those things you don't want changed within braces:
{Yb}.  Because of this problem it is common for journal bst's to avoid
the case munging feature, this leads to your problem!  You could try
modifying your _personal_ copy of osa.bst, but does that really solve
your problem since the journal presumably has the original style.  The
section in plain.bst that does the case munging is the following (see
the `change.case').
FUNCTION {format.title}
{ title empty$
    { "" }
    { title "t" change.case$ }
  if$
}
Personally I prefer to enter the title in sentence format, using lower
case where possible, and then avoid case munging.