Federal and Oregon law do not specifically address drug testing employees.  While drug testing is legal, an employer must follow a procedure that guarantees an employee’s constitutional privacy rights.  The following protocol is generally accepted as legal:

• The employer possesses a clear written policy regarding drug testing and it has been explained to all employees.
• The method of selection for drug testing is clearly explained, whether it is done randomly, as result of an employer’s reasonable suspicion (based on facts, not rumors), or because of a workplace accident or incident.
• The employer must apply a drug testing policy consistently, without discrimination.
• A 30-day advanced notice before drug testing is recommend to preserve an individual’s expectation of privacy, unless the employer’s written policy states differently.

The Americans with Disabilities Act (ADA) and Oregon disability laws do not protect an employee from discipline or termination if a drug test is positive, but they do protect recovering (non-practicing) alcoholics and drug addicts.

Source:

http://www.workplacefairness.org/drugtesting#2

http://www.oregon.gov/BOLI/TA/T_FAQ_Drugtesting.shtml