The root of the word "oppression" is the element "press". The press of the crowd; pressed into military service; to press a pair of pants; printing press; press the button. Presses are used to mold things or flatten them or reduce them in bulk, sometimes to reduce them by squeezing out the gasses or liquids in them. Something pressed is something caught between or among forces and barriers which are so related to each other that jointly they restrain, restrict or prevent the thing's motion or mobility. Mold. Immobilize. Reduce.
The mundane experience of the oppressed provides another clue. One of the most characteristic and ubiquitous features of the world as experienced by oppressed people is the double bind - situations in which options are reduced to a very few and all of them expose one to penalty, censure or deprivation. For example, it is often a requirement upon oppressed people that we smile and be cheerful. If we comply, we signal our docility and our acquiescence in our situation. We need not, then, be taken note of. We acquiesce in being made invisible, in our occupying no space. We participate in our own erasure. On the other hand, anything but the sunniest countenance exposes us to being perceived as mean, bitter, angry or dangerous. This means, at the least, that we may be found "difficult" or unpleansant to work with, which is enough to cost one one's livelihood; at worst, being seen as man, bitter angry or dangerous has been known to result in rape, arrest, beating and murder. One can only choose to risk one's preferred form and rate of annihilation.
Marilyn Frye, "Oppression," in The Politics of Reality, 1983