Store metakey value as an array

I don’t think WP_Query returns the meta fields for you as well. So you need to loop over the posts and run get_post_meta() manually. Assuming one office has at max one _octopus_id, you could write it like this:

// here we will store the ids
$octopiIds = [];

$query = new WP_Query([
    'post_type' => 'office',
    'post_status' => 'any',
    'posts_per_page' => 10,
    'meta_query'     => [
        [
            'key'        => '_octopus_id',
        ],
    ]
]);
while ($query->have_posts()) {
    $query->the_post();
    $octopiIds[] = get_post_meta(get_the_ID(), '_octopus_id', true);
}
wp_reset_postdata();

Do you really need the pagination of WP_Query? If not, I would suggest writing your own query via $wpdbs get_col(), as it can save you alot of queries. The code above will create at least 11 queries (1 for WP_Query, 1 for each post to get the meta), but you can do this in only one.

(untested)

global $wpdb;
$octopiIds = $wpdb->get_cols(
    $wpdb->query("
        SELECT DISTINCT pm.meta_value
        FROM {$wpdb->posts} AS p
        INNER JOIN {$wpdb->postmeta} AS pm
            ON p.ID = pm.post_id AND pm.meta_key = '_octopus_id'
        WHERE pm.meta_value != '' AND pm.meta_value IS NOT NULL
    ")
);

As an additional note, if you’re storing some kind of relationship between octopi and offices here, consider using a taxonomy instead of postmeta, as it will speed things up alot.

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