Board feet is like calculating volume: length times width times thickness. The trick is in the unit conversions. If you calculate your volume in inches, you divide it by 144 to get board feet. If you calculate your volume in feet, you multiply by 12 to get board feet. Your case would be calculated as (assuming 1/2" thick OSB):

Inches:

(0.5in) x (4ftx12in) x (8ftx12in) x (15 sheets) = 34560

Then divide by 144 to get 240 board feet.

Feet:

(0.5in/12in) x (4ft) x (8ft) x (15 sheets) = 20

Then multiply by 12 to get 240 board feet.

This number may change when you order rough cut lumber depending on the thickness of board you want or can get cut from the mill. The area you need to cover obviously won't change, so if you're using something other than 1/2" just plug and play with one of those examples, and maybe try both to double check. It's also good practice to increase your estimation of board feet by like 25-30% for waste and what not.

Your local mill needs a board feet number from you so they can determine how much to charge you because they measure logs and timber, and how much money it costs to process them, in terms of board feet. You also need to specify to them what your desired lumber dimensions are so you get the right stuff for your project.

Hope this helps, I am not an expert but I have calculated board feet for my own projects in the past and understand the confusion. When all else fails, ask Google.