In essence, wrongful termination or wrongful discharge is when one is fired because of discrimination in the workplace. That runs the gamut from gender discrimination to retaliation for filing a worker's compensation claim to whistle blowing activities. Basically, if you are fired because you are in a protected class, that is wrongful.
That said, employment is also at will in Oregon. In fact, it is at will in most states. That means an employer can fir you or lay you off for any other reason, whether that reason is true or not. They can even fire you for no reason at all, and they do not have to tell you the reason.
This means, that most of the time when someone's employment is terminated, it is not wrongful in a legal sense. It is often unfair and unwarranted, but karma tends to be your only recourse.
To find out if your termination was wrongful, one really needs to consult with an employment law attorney. Even if there is nothing wrongful in a legal sense about the termination, attorneys are often able to find other issues the employer handled incorrectly, such as your final paycheck, and often times the attorney is able to recover something for you for those other violations.
Lastly, this and everything above applies largely to private employers and non-union jobs only, as those have different rules that generally are more favorable to employees.