This is usually because of your role permission. Your role decides which users' opportunities you can access. You only see opportunities owned by users inside that access, so if you move an opportunity to a user outside it, you lose access to it yourself.
The same setting covers orders. There is no separate access setting for orders.
Contact your admin to change your settings. An admin changes it under Settings → Account → Users & roles: open your role and change the Access level for opportunities under Set sales role access.