OID - constantly increasing/ what about the gaps?

Hi,

A simple, almost stupid, question :

Does the OIDs allways, without exception no matter what I do, increase? 
If I insert a lot of records, delete some of them, inserted more the OID would be :

( about ) 18000 + number of inserted + more records

I mean, the gaps from the deleted records won`t be filled? So I can use this as a measure for new records for example. I could keep track of OIDs when users access a page generated with data from my database, when they return I give them all the rows with OIDs higher than the one I stored at their last visit. 

Do I make sense? Damn, I`m tired!

Thanks anyway.

Thomas