Lasse, <!--more--> tag сurrently works only for the blogs' list.
When blog posts are added to newsfeed, "more" tag is ignored and blog gets cut by 150 symbols.
150 symbols limit is set in init.php ( ow_plugins/blogs):
1. function blogs_feed_entity_add : $content = nl2br( UTIL_String::truncate(strip_tags($post->post), 150, '...') );
2.function blogs_feed_entity_update: $content = nl2br( UTIL_String::truncate(strip_tags($post->post), 150, '...') );
You can customize the code yourself to fit your needs.
Oxwall doesn't use "more" tag when adding newsfeed items since not all users will use "more" tag in the right place. For example users might add it toward the end of the long blog post and your newsfeed will look even more messed up.