Change the "Recent Posts" title in Opti to "Recent Articles".

opti-translate-recent-posts.php

<?php
/**
 * Plugin Name: Opti change "Recent Posts"
 * Plugin URI: https://prothemedesign.com
 * Description: Change the "Recent Posts" title in Opti to "Recent Articles".
 * Author: Ben Gillbanks
 * Version: 1.0
 * Author URI: https://prothemedesign.com
 * Text Domain: broadsheet
 */

/**
 * Change the translatable text to something else.
 *
 * @param string $translation The translation to use.
 * @param string $text The text string being translatec.
 * @param string $domain The theme domain being used.
 * @return string
 */
function opti_get_text( $translation, $text, $domain ) {

	// If we're not using Opti then quit.
	if ( 'opti' !== $domain ) {
		return $translation;
	}

	// Adjust homepage string.
	if ( 'Recent Posts' === $text ) {
		return 'Recent Articles';
	}

	// Adjust paged variations.
	if ( 'Recent Posts - page %d' === $text ) {
		return 'Recent Articles - page %d';
	}

	return $translation;

}

add_filter( 'gettext', 'opti_get_text', 10, 3 );