How are the themes of The Perks of Being a Walfower and the poem "O Me! O Life!" alike and how are they different?

One similarity between the themes of Whitman's "O Me! O Life!" and The Perks of Being a Wallflower lies in how both affirm the possibility of happiness.  
Both works highlight the struggle to find happiness.  In both, this struggle is set against a world filled with insecurity and doubt.  In Whitman's poem, the speaker is immersed in uncertainty.  "Recurring" questions, "endless trains," and "cities fill'd with the foolish" underscore the difficulty to find happiness.  In much the same way, Charlie is scared of his first day of high school. He is completely overcome with how hard it is to find happiness in a setting where everyone else seems happy except for him.  In both works, questions and the lack of certainty define the world.
However, the overriding theme in both works is that we can make peace with the conditions around us and still find happiness.  Whitman believes that his answer to the uncertainty of the world is that "you are here" and "that life exists."  For Whitman, the ability to "contribute a verse" to "the powerful play" that is life is where happiness lies. Despite all that might surround him, contentment can be found in his ability to live and be a part of the world. Charlie comes to this same realization towards the end of The Perks of Being a Wallflower:

So, if this does end up being my last letter, please believe that things are good with me, and even when they’re not, they will be soon enough. And I will believe the same about you.

Like the speaker in Whitman's poem, Charlie believes that he will find happiness.  Both works stress how individuals do not have to take the form of the world around them in order to be happy.
A significant difference between the themes of both would lie in their social context.  The speaker in Whitman's poem experiences his revelations about happiness through his own introspective musings.  Charlie is able to come to his understanding because of the help of people outside him. Figures like his friends and teacher play critical roles to help him him gain the insight he needs to find happiness.  While this difference is significant, it does not diminish from the theme in both works that people can find happiness in a world that might contain significant amounts of unhappiness.