O'Brien is removed from the front lines when what happens to him? also who do the US soldiers most frequently encounter as enemies?

O'Brien is removed from the front lines when he accepts a job at the rear. He is airlifted away from the front lines to perform his new duties away from the fighting.

The second part of your question deals with enemies; those frequently met and oh so hard to detect enemies were in fact..... land mines.

After months of lobbying at the battalion firebase for a job in the rear, it finally comes through, and he is given a job in the battalion headquarters', having served in a rifle platoon less than six months. He is assigned to the Personnel & Administration section (S-1) of the unit.

WHO do the soldiers most frequently encounter as the enemy? On some level, every sullen, antagonistic, anti-American Vietnamese they see; villagers--old men, women, children, water buffalo, everything Vietnamese. In the jargon of the GI, "gooks". While non-combatants, they are considered by the GIs as a visible manifestation of "the enemy". Our perception and treatment of them as such reinforced their animosity toward American GIs. It was a self-perpetuating cycle.

If the question is, WHAT aspect of the enemy accounts for the most injury and death to the GIs, that would be: mines, booby traps (currently referred to by the military as Improvised Explosive Devices [IEDs]), and the occasional sniper round.