obligate a. 有责任的；必需的 

obligation n. 义务；职责，责任

obnoxious [ob-=against 反 ; noxi = harm 伤害;-ous a. = having 有…的=>“having harm against 对…有害的”→]a. very disagreeable; offensive; hateful 令人非常不快的；令人讨厌的；反感的 / His disgusting manners at the table made him obnoxious to us. 他在餐桌上令人作呕的举止使我们都非常讨厌他。

obnoxiousness n. 令人讨厌

obstruct [ob- = against or in the way of 挡住;struct = to pile 堆积=> “to pile in the way of 堆积以挡住…去路”→]v. block; close up 阻塞;堵塞 / Fallen trees obstructed the road. 倒下的树木把路给堵了。

obstruction n. 阻塞；阻挡；障碍物 

obstructive a. 引起阻塞的；阻挡的，妨碍的

obvert [ ob- = toward 向；vert = to turn 转 => “to turn toward 转向…”→]v. turn (sth. ) toward an object 将…转向某物体 / He obverted the mirror to the sun. 他将镜面转向太阳。

obverse n.  正面；较显著面      a.  正面的；显著面的

下面是ob-异体oc-，of-, op-的几个构词实例。 

occur [oc- (ob-) = toward 向；cur = to run =>“to run toward 向 …跑来”→] v. appear; be found 出现，存在 / “E”occurs in print more often than any other letter. 字母E在印刷品中比其他任何字母都常见。② take place, happen发生/ Delays are liable to occur. 可能发生延误。

occurrence n. 发生；出现；事件