I want a name translated but not sure which to pick, what sounds the best and what's the difference between them? The name which I'm trying to translate is "Oter".
My recommendation is オター. That horizontal bar after タ is a long vowel marker, which indicates the vowel of the "ter" part should be pronounced longer than usual. Usually Japanese people prefer a long vowel for the "-er" and "-ar" sound at the end of a short name, for example コナー "Conar", ケラー "Keller". In addition, オタ is often used as an abbreviation of オタク, so it may carry an undesirable connotation.