Some accents are plainly ignored by the LM hash algorithm, but never by the NT hash algorithm. When we finish cracking a LM hash we then try all possible accents until we find the password that matches the NT hash. With ophcrack 2.4 the following accents are supported:
È Ê Ë è ê ë Ù Ú Û ù ú û À Á Â Ã à á â ã Ò Ó Ô Õ ò ó ô õ Ì Í Î Ï ì í î ï
If there are too many combinations (e.g your password is made of 14 o's) ophcrack 2.4 may reduce the set of accents it tries.