What does the French expression OUILA mean - is it merely a different spelling for VOILA?

"ouila" does not exist in French, maybe you mean "oui, là" (yes, there) which is about as much as the English expression is used, as it is not really an expression.

A close synonym to "voilà" (here comes) is "voici", but it is rarely used. It originally had the same distinction as between "there" (là) and "here" (ici), but it is now blurred.