The day you are scheduled to take your OA is a very important one. Make sure you dress sharp. You are going for the interview of a lifetime. An interview that can lead you to your dream career and to the world (literally).
What to Wear
You need to wear a suit. Do not just go in with a shirt and tie. You need an entire suit. Do not wear khakis. This is business formal, not business casual.
For males, think about what you want to project when selecting your tie. I recommend only choosing from one of four colors: Red, Blue, Yellow, Silver.
Red signifies power and confidence. Blue represents peace and diplomacy (hey now!), Yellow gives the impression that you are intelligent and fun. Finally, Silver can give the impression that you are full of potential.
Don't wear black. It says nothing really. Any other color I wouldn't recommend either.
I would look into wearing a tie clip as well. Nothing distracting, just subtle. What a tie clip does is say "Hey interviewer, look how much pride and detail I have in my appearance. I will exhibit the same in my work".
Getting to the OA
Most OAs are held at 7am (sharp!), be in the building by 6:30am. If your OA is scheduled later in the day arrive 30 minutes early. It gives you time should something come up.
My OA was on 301 4th St SW. The closest Metro stations were L'Enfant plaza (Yellow/Green lines) and Federal Center SW (Blue/Orange lines). After exiting the Metro station it is literally a 4 block walk.
If this is your first trip to DC it is HIGHLY recommended to make a dry run the day prior to your assessment. The last thing you want to do is arrive late because you thought you knew where it was and were wrong.
What to Bring
After you complete your SF86 on eQip certify it but DO NOT release it. What happens is when you certify it it then allows you to print not only the SF86 but the 4 additional release forms that come with it. If you do not certify it you can only print a copy of the SF86 and not the 4 release forms you need to sign.
I did this wrong, so they fingerprinted me, gave me the cards and I had to mail everything back to them (probably delaying my security interview).
Also bring your invitation letter, just in case your name is not on the list. I don't see that being a problem but it's always good to be prepared.
Foreign Service Officers may need to bring an additional form, so double check your invitation packet.
The lobby is connected to a McDonalds. If you need coffee prior to your OA get it then and drink it. Be ready to go when they call you up.
The lobby is divided into 2 sections, one for FSO candidates and one for FSS candidates. Usually there will be about 4 times as many FSO candidates as there will be FSS candidates. On the morning of my OA there was only one other FSS candidate with me and she was applying for a different position (HR I believe).
Good luck to everyone applying for the Foreign Service. The day of your OA is a nerve-racking experience (for most) but can be a day that pays off big for you. The last thing I will say in this post is do not take it lightly. This is a very important day for you, so treat it as such.