This is a late issue but here it is

Whenever we enter "œ" (oe ligature) in a content object, the corresponding URL Alias contains "ae" (ae ligature) whereas it should be "oe".

The problem comes from the transformation table which does not map to the correct char.

Wikipedia page with some related information:
http://en.wikipedia.org/wiki/%C5%92#.C5.92_in_different_languages

U+0152 = Π= "OE", not "AE".
U+0153 = œ = "oe", not "ae".

more info:
http://www.fileformat.info/info/unicode/char/0152/index.htm

from eZ Publish (4.0 stable)/share/transformations/latin.tr:
<code>
snip
latin-exta_transliterate_ascii:
U+0132 = "IJ"
U+0133 = "ij"
U+0138 = "k"
U+0149 = "'n"
U+014A = "N"
U+014B = "n"
U+0152 = "AE"
U+0153 = "ae"
U+017F = "s"
snip
</code>