Custom fields empty after refreshing page

Try adding this save function to see if it saves data

function save_custom_artiesten_metabox_on_reload() {
    global $post;

    if (isset($_POST['oa_datum']) && isset($_POST['oa_aanvang']) && isset($_POST['oa_locatie']) && isset($_POST['oa_toegang_prijs'])) {
        update_post_meta($post->ID, 'oa_datum', sanitize_text_field($_POST['oa_datum']));
        update_post_meta($post->ID, 'oa_aanvang', sanitize_text_field($_POST['oa_aanvang']));
        update_post_meta($post->ID, 'oa_locatie', sanitize_text_field($_POST['oa_locatie']));
        update_post_meta($post->ID, 'oa_toegang_prijs', sanitize_text_field($_POST['oa_toegang_prijs']));
    }
}
add_action('admin_init', 'save_custom_artiesten_metabox_on_reload');

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