When I grew up, people always described their first name as their 'christian name', but in more recent times, the development of multi-cultural and multi-faith societies means that isn't relevant for a lot of people. So now you will be asked (particularly on official forms) for either, your 'first' name, or, more likely, your 'given' name. Given name tends to work better, because in some cultures, their 'first' name is actually their family name (what we would call a surname). So people have a family name (which they inherit, usually from the father, but not always) and a given name (or names) which the parents choose for them at birth, or at a later naming ceremony. So in your case, you have two given names, one Korean, and one not Korean. So it is up to you how you introduce yourself. If, as you say, you go by Stella and that is what you want people to call you, then just say, my name is Stella. If you are filling out an official form, you will probably need to use your Korean name? If someone looks at you oddly when you say your name is Stella, then you can choose to add that your Korean name is Oo, but you prefer to be called Stella.