Object Oriented Plugin not working

I’m trying to develop my WordPress skills a little. I’ve previously moved some of my functions.php that I use time and time again into a must-use plugin. I’m now trying to make that Object Oriented – with limited success. The code below is an excerpt of my overall code. The first section works perfectly, reducing the excerpt length, but the dashboard tidy doesn’t. Both previously worked as procedural code.

Any help in solving this would really be appreciated. Also, I’m struggling to understand why it is recommended to use the init rather than putting my actions and filters into the constructor. Can anyone explain this for me?

class OOtest
{
    public function __construct()
    {
        add_action( 'wp', array( $this, 'init' ) );
    }

    public function init()
    {
        //Tidy up the dashboard
        add_action('wp_dashboard_setup', array( $this, 'tidy_dashboard') );

        //Edit excerpt length
        add_filter('excerpt_length', array( $this,  'custom_excerpt_length') );
    }

    public function tidy_dashboard()
    {
        global $wp_meta_boxes, $current_user;

        // remove incoming links info for authors or editors
        if(in_array('author', $current_user->roles) || in_array('editor', $current_user->roles))
        {
            unset($wp_meta_boxes['dashboard']['normal ']['core']['dashboard_incoming_links']);
        }

        // remove the plugins info and news feeds for everyone
        //Wordpress Development Blog Feed
        unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_primary']);
        //Other WordPress News Feed
        unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_secondary']);
        //Quick Press Form
        unset($wp_meta_boxes['dashboard']['side']['core']['dashboard_quick_press']);
        //Plugins - Popular, New and Recently updated WordPress Plugins
        unset($wp_meta_boxes['dashboard']['normal']['core']['dashboard_plugins']);
    }

    // Add custom excerpt length
    public function custom_excerpt_length($length)
    { return 55; }
}
$foo = new OOtest;
?>