To cancel your order, you may call OVH support OR send an email to
support@ovh.co.uk (Or corresponding OVH country).
BTW, remember OVH staff only works on office hours, so you won't get any answer before tomorrow. Server installations are performed also on office hours, so, if you ordered your server on friday, the earlier you will get it is on monday. Finally, if your account is a new one, you may need to validate your identity. If so, you may have received an email on that already.