Common Words
* oblige: to force or require someone to do something.
* object: to express disapproval or opposition.
* obscure: difficult to understand or see clearly.
* obvious: easy to see or understand.
* obese: excessively overweight.
* obstacle: something that blocks one's way.
* obstruct: to block or hinder movement or progress.
* obtain: to get something, usually by effort.
* obstinate: stubbornly refusing to change one's opinion or course of action.
Less Common Words
* obliterate: to destroy completely.
* oblivious: unaware of what is happening around one.
* obnoxious: extremely unpleasant or offensive.
* obscene: offensive or disgusting.
* obsess: to think about something constantly.
* obsolete: no longer produced or used.
* obtuse: slow to understand or lacking in intelligence.
* obviate: to remove a need or problem.
* obverse: the front or face of something.
Words with "ob" as the root
While less common, there are some words where "ob" is actually the root, not a prefix:
* ob: a unit of measurement for the rate of absorption of light.
Let me know if you'd like me to provide more examples or explain any of these words in more detail!