I know of no law regarding sheduling in Oregon. However, given that they wrote you up for what appears to be an unreasonable excuse, there likely is another underlying cause. For instance because of race, gender, wage claim, arguments between you and management, etc. The likely more relevant question is whether the true underlying reason for the write up is lawful.
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Assuming you are not a member of a bargaining unit subject to a collective bargaining agreement between the union and management, but rather an at-will employee, I see little you can do. You do not suggest any facts that suggest unlawful discrimination or unlawful retaliation, so what I am left with is an employer that did something unwise (changing your schedule with inadequate notice) and then penalized you for it unfairly by writing you up for it. Unfortunately for employees, there is no law that requires employers to conduct their business affairs wisely or to treat their employees fairly. Employers are legally entitled to do dumbass things and then to unfairly blame the inevitable result on hapless employees. My condolences that this happened to you.
Not legal advice as I don't practice law in Oregon. It's just my two cents on the facts you describe in light of general principles of law. If you need legal advice, please consult a lawyer who holds Oregon licensure. That's not me as I practice in Vermont ONLY.
You are describing an unfair situation, but whether it is illegal could depend on your employer's motivation. If the schedule change followed by discipline was in retaliation for your engaging in protected conduct, such as protesting the lack of breaks, working overtime without pay, or the use of medical leave, to name a few, then your employer's conduct is likely unlawful. But if your employer's motivation was to obtain needed coverage, and it was merely negligent or unreasonable in the manner of communicating with you, this is not illegal. There are many unfair work situations that the law simply cannot protect against. You should review the employee handbook and policies on shift change notifications and obtain any needed clarification as to those policies and your employer's expectations. An employer's inconsistent application of its own policies is often evidence that its conduct was unrelated to legitimate business needs--but again, that inconsistent application must be based on protected status or protected conduct to be actionable as an unlawful employment practice.
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