@oristarA,
2. Marines are "fancy-pants" because they are seen as of higher status than ordinary infantry. Fancy-pants and fancy-schmancy can mean fancy clothes, but here it means they think themselves better.
3. I don't know what OD stands for here, but he means his uniform.
4. American infantry soldiers are known as dog-faces. I don't know why, but when I was searching for the reason, I came upon "dog-face" as an expression used to describe the expressionless faces assumed by prisoners. Not sure if the two are connected...but I guess the army is a lot like prison in terms of obedience to authority and discipline and not wanting to draw the attention of authority to yourself.
5. Yes...pretty much.