Octalink isn't any different from any other English or Italian threaded bottom bracket, so you'd need a BB90-to-BSA adapter. Unfortunately, BB90 is wider than BSA, so I don't think there's any such thing.
I'm a little confused, however. I was under the impression that Octalink was only used by Shimano on 5500, 6500, and 7400 cranksets, and nobody else was allowed to use their BB standard. FSA helped create the ISIS standard to compete with Shimano's closed Octalink standard. Very few cranksets use either of these anymore.
If your FSA crankset has the spindle attached to the crankarms, then you don't actually have Octalink or ISIS and all you need is this:http://www.bikepedia.com/PA/Item.aspx?I ... VW-51csCk0
If you do have Octalink or ISIS, you simply won't be able to use the cranks and there's no adapter due to the BB shell of BB90 being wider than an entire BSA bottom bracket.