Heh! I am also sortof in your shoes. I volunteered to be a backup photographer and suddenly he decides that he does not want a main photographer and that I am it.... (^_^;;
Scary! But he is willing to take the risk and so this Saturday I shall have my 1st advanture in this area. I am pulling another guy in as a backup.
Here's my check list which I shall share with you:
1) Main camera and one backup camera in case anything goes wrong
2) 2 sets of batteries for camera
3) 2 sets of batteries for flash
4) Charger for batteries
5) Spare memory cards
6) Lenses: I shall be bringing these few - 18-55mm, 50mm, 28-70mm, and 55-200mm. (ur... for your olympus, I guess you will just have to do with it unless you want to add a teleconvertor to extand the reach... in which case I have one which I can lend)
7) External flash
8) Xdrive or Nixvue kinda storage device
9) At least one buddy in case you need some moral booster
Some events to take:
1) Bride combing hair and makeup before meeting the groom
2) Groom arriving in car and opening door (if natural shot no good you may wanna ask him to pose a bit)
3) Groom at door begging to see bride
4) Ceremony stuff at bride's place
5) Tea ceremony
6) Outdoor if required
7) Signing in for dinner
8) Walking in for dinner with kids at side and throwing flowers etc
9) Speech
10) Champagne + yam seng
11) Cake
12) Table to table
13) Sabo!!!
You backup probably will take those other events happening like
friends and jie mei stuff.
Settings, try to bounce flash if possible, if not use a omni bounce. If day is bright enough, use ambient and maybe a little fill flash. At dinner: If shutter speed too high the background will become black. If shutter speed too low you may get blurred images. See which is the best for your camera. Other stuff would be is your friend expecting prints or CD or ???.
Urrmmm.. that's all I can get out of my mind now. Oh... if you need a backup for yours, just PM me.
Good luck!