ol children instead of ul children

I’ve created a new walker class to display a custom layout when using the wp_list_comments(); function and this is perfect, the only thing is that all replies to one comment are listed in a <ul> – <ul class="children">. I would like these to be in an <ol> bracket. Is there any way to change this using something like start_lvl?

Heres my walker class – <?php wp_list_comments('type=comment&callback=letsfixit_comment'); ?>

function heres_my_comment($comment, $args, $depth) {
        $GLOBALS['comment'] = $comment;
        extract($args, EXTR_SKIP);

?>
        <li>
         <div class="oneComment" id="div-comment-<?php comment_ID() ?>">
        <div class="avatIcon pull-left">
        <?php if ($args['avatar_size'] != 0) echo get_avatar( $comment, $args['avatar_size'] ); ?>
        </div>
        <span class="author"><?php printf(__('<cite class="fn">%s</cite> <span class="says">says:</span>'), get_comment_author_link()) ?></span>
        <span class="date"><?php /* translators: 1: date, 2: time */ printf( __('%1$s at %2$s'), get_comment_date(),  get_comment_time()) ?></a><?php edit_comment_link(__('(Edit)'),'  ','' );?></a></span>
        <?php comment_reply_link(array_merge( $args, array('add_below' => $add_below, 'depth' => $depth, 'max_depth' => $args['max_depth']))) ?>

        <?php if ($comment->comment_approved == '0') : ?>
        <em class="comment-awaiting-moderation"><?php _e('Your comment is awaiting moderation.') ?></em>
        <br />
        <?php endif; ?>

        <div class="comment-meta commentmetadata"><a href="<?php echo htmlspecialchars( get_comment_link( $comment->comment_ID ) ) ?>">

        </div>

        <?php comment_text() ?>

        <div class="reply">

        </div>
        </div>
<?php
        }