The standard booking office (OPC) assessment is the TSVT - Ticket Selling and Verbal Test.
There's also a basic maths and basic English test.
Very straightforward and passable for most people on here. The hard part is the interview in my opinion, but generally aslong as you get managers interviewing from the departments (ie Retail, Booking Office etc, and not HR people) you'll find they try make the interview as easy and straightforward as they can.