词源：
前缀ob-来源于拉丁介副词ob，原意是“toward”，在构词当中又引申出“against,inversely”等意思，还有“to, in the way, over, away, completely,intensive”等含义。ob-是个原生词缀，一般与拉丁词根缀合，所构成的派生词的词性取决于词根或后缀的属性。由于语音的同化作用，ob-在c,f,p前面，变成oc-,of-,op-。在元音字母及其它辅音字母前，ob-形态不变。
每个前缀的纯英语来源解释：
ob-
prefix meaning "toward, against, across, down," also used as an intensive, from Latin ob "toward, to, over against, in the way of, by reason of, about, before, in front of," from PIE root *epi, also *opi "near, against" (see epi-).
oc-
assimilated form of ob- before -c-.
of-
assimilated form of ob- before -f-.
op-
assimilated form of ob- before -p-.
os-
frequent form of ob- before -c- and -t- in words from Latin.
现在对这几个前缀的含义整理如下：
1.逆着…，和…敌对，和…相反
oppress, opprobrium
2.向着…，给…
obnoxious, opportune
3.妨碍地
officious, obsequious, obsession
4.越过，盖住
obscene, obfuscate, obscure
5.远远地，消失
obviate, omission
6.表强调[完全，干脆，厉害地，继续]
obese, obdurate, obeisance
另外，前缀ob-还有很多变形。
1.oc-[在辅音字母c之前]
occult, occupied