Implementing an OR statement to wordpress wp_query

You can run two queries and merge the results:

$cat_query_args = array(
    'paged' => $paged,
    'order' => 'DESC',
    'meta_key' => 'android_link',
    'post_type' => array( 'post', 'app' ),
    'cat' => '63',
);

$meta_query_args = array(
    'meta_query' => array(
        array(
            'key' => 'android_link',
            'compare' => 'EXISTS',
        )
    )
);

$cat_query  = new WP_Query( $cat_query_args  );
$meta_query = new WP_Query( $meta_query_args );

$merged_query_args = array_merge( $cat_query, $meta_query );
$merged_query = new WP_Query( $merged_query_args );