Six layers of 1/8 will work fine.  That's what I did. 

You do need to worry about springback and make sure that the OUTSIDE RADIUS of your laminated deckbeam on the form is about 1" less than the radius you want.

The finished radius refers to the outside radius.

Don't do what I did.

I needed a deckbeam with a 16" radius so I made a form with a 15" radius.  Piling several layers of the 1/8" material on that gave me an outside radius on the form of about 15.75".  When I removed it from the form, it sprung back to about a 17" radius.  I had to make a new form with around a 14" radius and it worked fine..

I used 3/4" x 1 1/2"  western red cedar, because it is light weight and I had some leftover from my sheer clamp.  Cedar bends really nice, I am not sure how easy oak will bend. 

As stilljustseven explains, I also had to adjust my form radius and bend two beams before I got the correct radius.

Jeff

Oak will bend fine, but the cedar is a better choice for a boat.  Personally I don't like the deck beam, especially at the cockpit area, so I removed it after the deck cured.  I've never had any issues without it. 

1/8 inch ply will work as well, just like "stilljustseven" mentioned.

FrankP

I got real lucky with mine.  outside radius needed to be 16" rad. so I built my form with a 14" rad.  I used 4 strips of 1/4" thick sitka spruce and it came out exactly to 16" outside rad.  When (notice I said "when"... not "if")   I am going to use thinner strips to make the bend easier to apply.  my forearms were sore the next day with all the grunting I had to do!

You can also lay out 3 or 4 curved shapes on a piece of plywood, cut them out and glue them together and have the laminations verticle rather than horizontal, or even cut one beam from a 3/4 ply.  You use up more material but know exactly what radius you will end up with. 

If as some have done, you want to use a temporary deck beam and remove it later the material you use can be almost anything.  Just remember to put plastic tape where you do not want it to stick.

I sort of feel comfortable knowing the strength of the deck beam is there if I have to hoist the bow of someone else's kayak over mine in an assisted rescue.  It also makes me comfortable sitting on the deck while eating lunch ashore.

Ed

I used 1/2" fir ply and cut it in the vertical position. No bending or radius problems. Just be sure to seal the crap out of it.

I'll admit that my MC16.5 weighs 85 lbs instead of the 62 in the plans. 

(weight with seats and 3 coats of paint on the hull)

I would think that the horizontal laminated construction would be a lot more durable than the vertical laminations due to grain direction of the wood, especially at the ends.  I think I'll try 1/8" [3mm] okoume strips if I have enough scrap...I want to roughly cut the top deck first to make sure I've got enough scrap material.  So, 'frankp', you just removed the deck beam after glassing the deck and didn't have any issues?  I wouldn't mind shaving any weight possible out of this boat.  I'm aiming for 16 feet long at 25 lbs if I can keep my epoxy from getting too thick.