if there is only one post in the category, directly open

this function will check if you’re on a category page and 302 redirect to the latest post in that category. put it in your theme’s functions.php file.

function my_check_if_cat(){
    if ( is_category() ) :
        $category = get_the_category();
        $latest = query_posts('showposts=1&cat=".$category[0]->cat_ID);
        if(have_posts()) :
            wp_redirect(get_permalink($post->ID), 302);
        endif;
    endif;
}
add_action("template_redirect','my_check_if_cat');