Obtain means to get or acquire. It begins with the letters ob.
Observe
Observe
The word is obscure.
The prefix od- otherwise known as ob- means toward, against, or completely.
oubliette
The word "obscure" means to hide or conceal something from sight. It comes from the Latin word "obscurus" which comes from "ob", which means toward, and the root "skeu", which is Indo-European for cover or conceal. So "ob" in "obscure" basically just means toward.
obviously
obsolete
opposed That starts with "op" not "ob"
To put an obstacle in the way
observeobsessobscureobnoxiousobsoleteobstacleobliqueobtuseobject, objective, objectionobtainobliterateobviousoblivionobsoleteobsceneobdurateoblong
starts at Maryland and ends at Kingston