Ad-hoc testing is done due to Lack of time or Resorces,
it contains various testing techniques like Munky testing
(this means testing importent testcases related to the 
functionality etc),Buddy testing etc...

Where as One time testing means Testing the total 
Application for the first time,it means not done the 
Retesting and Regression Testing.

HI
   ad-hoc Testing can be done with domin Knowledge (or) 
Domain Experts.

                     Where as

One Time Testing can  also be done with Domain experts 
only.

In general test engineeers conduct functional & system 
testing in our organization.some times it is not able 
conduct complete functional and system testing due lack of 
time,lack of resources,lack of skills,lack of 
knowledge,lack documents.
this type of testing is known as ad hoc testing.
Ex:monkey/champangi testing,pair testing,exploratory 
testing,buddy testing.
where as one time testing is conducting testing first time 
on AUT/build is called one time testing or build 
verification testing

Ad-Hoc testing will do if the requirements are not clear or 
don't have time to test complete testing. Ad-Hoc teting 
means testing an application based on previous testing 
knowledge.

One_time testing means testing an application only one 
time. it means not done the Retesting and Regression 
Testing.