This is a particularly interesting question to consider. The phrase hamartia refers to a tragic flaw in the hero that brings the tragic ending upon him or her. Thus it can be argued that Othello's tragic flaw is his jealousy. It is this that allows him to be exploited by Iago and brings the tragedy to its shocking conclusion. The problem with applying this term to this play is that the two title characters do not really have a tragic flaw in the same way. In fact. it is necessary to look at other characters to find the tragic flaw that is the cause of all the deaths.
Arguably, it is Friar Lawrence who has the hamartia of pride. At every stage of the play, it is he who prevents the revelation of Romeo and Juliet's marriage. It is clear that he desires to be the person who can create peace between these two feuding families, and the marriage of Romeo and Juliet is his strategy on how to achieve this. Note how he counsels Romeo in Act III scene 4 after he comes to him having killed Tybalt. Advising him to go to Mantua, the Friar says:
...thou shalt live till we can find a time
To blaze your marriage, reconcile your friends,
Beg pardon of the Prince, and call thee back
With twenty hundred thousand times more joy
Than thou went'st forth in lamentation.
At every stage, Friar Lawrence waits to reveal the secret of the wedding, trusting that there will come a time in the future when it can be decisively acknowledged, and thus bringing about a happy ending. It is hard to escape the fact, however, that had Romeo and Juliet revealed their marriage from the very first moment, so much tragedy would have been avoided. It was the pride of Friar Lawrence therefore that was responsible for the tragedy in this play.