$args = array(
        'posts_per_page' => -1,
        'post_type' => 'cpt_program',

        'meta_query' => array(
            array(
            'relation' => 'OR',
                array (
                    'key' => 'prog_ongoing',
                    'value' => true,
                    'compare' => '=',
                ),

                array (
                    'key' => 'prog_date_start',
                    'value' => date('Ymd'),
                    'compare' => '>=',
                ),
            ),
        ),

        'orderby' => 'prog_date_start',
        'order'   => 'ASC',
    );

// posts_orderby filter

function order_by_custom_fields( $orderby ){
    $start_date = date('Ymd');
    global $wpdb;
    if( ! is_admin() ) :
    $orderby =
        "CASE
            WHEN {$wpdb->prefix}postmeta.meta_key = 'prog_date_start' AND {$wpdb->prefix}postmeta.meta_value >= $start_date THEN '1'
            WHEN ({$wpdb->prefix}postmeta.meta_key = 'prog_ongoing' AND {$wpdb->prefix}postmeta.meta_value) AND ({$wpdb->prefix}postmeta.meta_key = 'prog_date_start' AND {$wpdb->prefix}postmeta.meta_value) THEN '2'
        END DESC";
    endif;
    return $orderby;
}