Output array into one table cell

Alternatively, make an array of the titles and then use implode…

$prerequisites = get_post_meta($post->ID, 'opl_prerequisites', TRUE);
$titles = array();
foreach( $prerequisites as $prerequisite ){
  $titles[] = get_the_title( $prerequisite );
}

$output .= '<td>'.implode(",", $titles).'</td>';