One way to hold this word: picture a canvas with paint on it thick enough that you can still see where the brush went. Oil paint stays wet for days, so the painter keeps returning to the same spot, layer over layer, until it is finished. That slowness is why an oil painting is, in English, the most 'finished' kind of picture — the kind you frame and hang. From that one picture the word reaches in two directions: out to the craft that makes it, and sideways into a British joke about a face worth framing.
Two branches
畫出來的那幅畫，和畫它的那門技術
同樣兩個字，一邊指看得見、摸得到的成品，一邊指做出它的過程。英文靠可數不可數把兩者分開：加 a、可以數的是那幅畫；不加 a、不加 s 的是那門技術。
noun · B1
A picture that someone has made with oil paint — the thick, slow-drying kind of paint artists use on canvas. The words name the finished picture itself, so you can count them.
油畫（作品）
拿「值得被畫下來」當好看的標準（英式玩笑）
核心畫面裡的油畫是會被裱框掛起來的東西，所以「值得被畫成油畫」就成了好看的標準。這個聯想是理解上的橋，不是字源上的來源。
noun · B2
A joking, indirect way of saying that someone is not good-looking. It is almost always used in the negative — no oil painting — and very often about yourself. The source marks it as British and humorous.
長得不好看（幽默的說法）