You don't need to be an expert on spelling, grammar or syntax (OUP has resources to handle those issues), but you do need the ability to tell whether an article is a good one or the other sort. There is no fame or glory for doing the reviewing; nor is there any pay. On the plus side, your identity is kept away from the author, who will never know who it was who recommended turning your article into a fleet of paper boats.
This appeal may be repeated from time to time. This is because peer reviewers generally burn out after they're reviewed three or four articles. This is quite normal, so if you do volunteer to review copyright articles you needn't feel that you are wedded to the commitment.