Nested IF To Enter Information Into Column 'A'

I am trying to use a nested IF to enter info into col A. It is based on 4 pieces of info, I can get the first 3 to work but am struggling with the last part. The problem is that I am looking at the same cell for the first 3 parts but the last part is looking at another cell reference. A nested IF may not be the way to go. An example of the spreadsheet is as follows:

A B C D AB
1 12/1/10 Comp 1 Surveyer 1 18/1/10
Omit 11/1/10 Comp 2 Surveryer 1 Omit
0 10/1/10 Comp 1 Surveyer 2
Comp 3 Surveyer 1

The formula I am currently using in column A is:
=IF(AB3="Omit","Omit",IF(AB3,1,"0"))
This works for the first 3 pieces of information but however does put a 0 in col A if there is no date in col AB, but this what I expect as it's what I've asked for. I am missing the last piece of information.

But what I want to put in column A is:
If there is a date in column AB - enter 1
If there is the word 'Omit' in col AB - enter Omit
If there is no date or the word Omit in col AB - enter 0 and finally
If there is no date in col B - leave blank (if there is no date in col B, col AB will always be blank)