Does the client, alone, determine if the OD consultant should be hired?

Yes, clients pay the bills, so they get to decide who the consultant is$.$

Yes, clients know their system best and have the best vantage point to determine the appropriate fit.

No$.$ There is a licensing body that determines which area an OD Consultant can work.

No$,$ the OD Consultant bears the burden of responsibility to seek an appropriate match between her$/$his skills and the needs of the client.