This answer assumes you are seeking the Spanish pronunciation. I have no idea how Basque pronounces anything.
If there are no accent marks, then it would be o-REN-dain (o-REN-dine in English spelling), with accent on middle "REN." This is because in Spanish the next to the last syllable takes the stress in any word that ends in a vowel, "n," or "s." If the word ends in a consonant, then the stress is on the last syllable. The only way to change that is to use an accent mark to indicate the syllable that should have the accent.

If Oredain were to be pronounced with stress on the final syllable, o-ren-DAIN, then you would have to place an accent mark on the "a." The letters "e" and "i" are "weak" vowels in Spanish and so glide into and form a diphthong with the vowel next to them. Following this rule, "dai" would be pronounced like English "die" (setting aside for the moment the fact that Spanish "d" sounds more like English "th" and not English "d"), regardless whether it is the syllable that is stressed or not. If you wanted the final "i" to have the stress, then you'd have to put an accent mark on it.

As to what the original spelling with accent marks was, that might be mostly conjecture since Spanish famously did not start to use accent marks regularly prior to about two centuries ago.

If you can get away with it, just put Orendain in all caps ("ORENDAIN"). Then you can sidestep the whole accent mark question, since Spanish capital letters do not take diacritical marks.