meta queries do ‘either/or’ checks

meta_query works on ‘both/and’ checks and also ‘either/or’.so don’t worry.your coding was correct.this is the actual format…

$a = new WP_Query(
array(
    'fields' => 'ids',
    'ignore_sticky_posts' => true,
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key' => 'foo',
            'value' => array( 'foobar' ),
            'compare' => 'IN'
        ),
        array(
            'key' => 'bar',
            'value' => array( 'foobar2' ),
            'compare' => 'IN'
        )
    )
) );
print_r( $a->posts );

---

you should try with this coding…

    $args=new WP_Query(
     array(
    'post_type' => 'post',
    'posts_per_page' => -1,
    'meta_query' => array(
    'relation' => 'OR',
        array(
            'key' => 'app_os',
            'value' => array('iPhone'),  
            'compare' => 'LIKE',  
        ),
        array(
            'key' => 'app_os_2',
            'value' => array('iPad') ,
            'compare' => 'LIKE',  

        )
    )
    ); 
print_r($args->posts);