I have been in the process of making a bathroom cabinet. The countertop is made from 2" oak and is 22" wide and 60" long. heres the problem. i started with 4 pieces of oak about 6" wide and alternated the growth rings and glued them together.. all was good until I cut out for the sink. Now the couther top is starting to cup.. How can I flatten this back out?

Sorry to say but there isn't much you can do at this point. The stresses in the grain have taken over. If the cupping is not too great you might be able to glue and screw the top to the cabinet and pull it back to flat. Wet the concave side of the wood and this will cause that side to swell slightly compared to the convex side.

On the underside install 20" long 2x2" cross members of oak on each end and on either side of the sink hole. Attach these by screwing in the center and each end, however the end screw holes (in the cross piece) will have to be slots or elongated to allow for wood movement. When installing the cross pieces clamp each end to to the top to draw the counter top flat