Oelwein comes from the German words Oel "oil" and Wein wine"
Generally, the oil that is referred to is "olive oil".
Someone in the family hundreds of years ago was a producer or merchant of wine made from olive oil.
The word Oel was originally spelled with an "O Umlaut"... i.e., two little dots above the O. So if you look the word up in a German dictionary, you will find the word spelled with an "O" with two little dots above the O, and then followed by the letter "l".