No, you cannot use a pen and paper during the Online GMAT exam. The exam now comes with an online whiteboard. It can be used for making notes or solving problems during the exam. However, some people have expressed their concerns regarding the whiteboard. There may be a lag while using the whiteboard, and for people taking the online exam it may make them uncomfortable.
I would recommend that you practice using the whiteboard provided on the Gmatclub to get a feel of this new tool. I believe that practicing the use of online whiteboard will eventually make one comfortable. Further, this will reduce the probability of unwanted issues during the exam.
Note: The online GMAT exam issues result within 7 days.