The word Oriental is rooted in the concept of Orientalism, which is strongly articulated by Edward Said in eponymous book, Orientalism. In his book, Said illustrates the Orient as being the observed "other" or "foreigner," whereas the Occident (or the West) is centred as the default. All Eastern culture is prescribed through this imperialist, pejorative lens, where it is deemed as strange, exotic, dirty, effeminate, and weak. By presenting Eastern culture through this lens, Western culture may set itself up for dominance through cultural hegemony and colonisation.
Calling someone Oriental reinforces those hegemonic ideas even if you're unaware of its historical context.
By continuing use of this term, we are perpetuating anti-Asian sentiments and the centralisation of Western (i.e., Occidental) imperialism. Diverse Asian people, nations, and cultures are flattened into one caricature of itself, which is dehumanising.
- Asian or Asian-American when you're uncertain. However, when possible, be as specific as possible. If you know or can ask someone's ethnicity, it's better to be specific such as Korean-Canadian or Vietnamese-American.