According to the Comparative Tables, the primitive diphthong [au] (as well as [au] from primitive [ou]) first became rounded long [ǭ] and then developed into long [ō] in the Ossiriandic dialect, instead of being retained as it was in East Danian (PE19/25). In The Etymologies, however, this change did not occur for Dan. laur (Ety/LÁWAR), so perhaps Tolkien changed his mind about this rule.