There are many reference books out there that would help..The Navy's BM 3/2 and their Seaman book is good also.. Chapmans seamanship and small boat handling, the list goes on and on.....There is one website that I like to take practice tests on.. www.seasources.net ..The thing with all of these suggestions is that there is a lot more info than you will need to test for AB OSV...So what do you concentrate on? How will you know what will be asked?
It looks to me like you will test, modules 461XX Deck General and Navigation General also module 462XX Deck safety and Rules of the Road...Also a knot tying demonstration..
There are literally thousands of questions in the CG test bank on these subjects..Being you stated that you are applying for AB OSV. that would mean to me that your sea service would be between 180 and 360 days..Unless you have a really strong nautical background in that time, may I suggest taking an AB course in lieu of testing at the local REC..?..These courses are full of great information that is directed at what you will be expected to know as an AB..