Under the OCMB header there is a link to "Preferences", from there click "Reading topics" in the left menu, and in the middle of the page there is a setting for "Sort topics by", which lets you select your default sort method and order.
This sorts topics
only, not posts
List of topics
sort by default by the most recent updated topics at the top. Inside each topic the newest post
is at the end (so opposite). You can change the sorting of posts
at the bottom of each list of posts, but you can not save that change.