more. After being a dragon, Romeo is then a tyrant, an angel, a raven disguised as a dove, and then a lamb that appears to be a wolf. 

What can a reader or listener make of this shifting imagery? It depicts how Juliet is unsure of everything. She is confused and grasping at a solid understanding of Romeo's actions. By the end of the speech, Juliet has gone through many images but settled on the fact that Romeo may be a monster dressed as a god (or whatever image you want to go with). 

A thesis statement could be: Romeo's murdering of Tybalt causes Juliet to question all of her notions of Romeo, although, despite his actions, she still views him as beautiful.

You may want to explore a thesis statement such as, "In the monologue O serpent heart, William Shakespeare shows that Juliet is feeling conflicted about her feelings for Romeo."  Juliet has a powerful love for Romeo, yet she has also just found out that he has killed her cousin, Tybalt. While it may be expected that Juliet would not feel conflict because her hatred of Romeo that would result from his murderous actions toward her family would replace her love for him, this does not happen.  From the monologue, and Shakespeare's use of oxymorons such as "beautiful tyrant," "damned saint," etc, suggest that Juliet also has two opposing emotions existing simultaneously within her.  Her love never wavers, and yet it is not as if she ignores her feelings of anger for Romeo's murderous deed either.  The simultaneous presence of these emotions causes Juliet much conflict.

With this specific soliloquy in mind, it would most make sense to argue that Juliet was infatuated with Romeo. This particular soliloquy occurs in Act III, scene 2 after the Nurse led Juliet to believe that Romeo was killed. Once the loose ends are tied, it's clarified that Tybalt, Juliet's cousin, was in fact killed by none other than Romeo. Grief stricken, Juliet sends the Nurse to fetch Romeo so that they can celebrate their recent marriage. In a brief moment, Juliet has transformed from an excited bride to a "widow" and a young lady seeking the Nurse's advice to a woman capable of making decisions on her own.

The soliloquy you must refer to is full of oxymoronic expressions to describe Romeo. An oxymoron is when two seemingly contradictory terms coexist. I've taken the soliloquy you provided and bolded examples of oxymorons. 

O serpent heart, hid with a flowering face!
Did ever dragon keep so fair a cave?
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!
Despised substance of divinest show!
Just opposite to what thou justly seem'st,
A damned saint, an honourable villain!
O nature, what hadst thou to do in hell,
When thou didst bower the spirit of a fiend
In moral paradise of such sweet flesh?
Was ever book containing such vile matter 
So fairly bound? O that deceit should dwell
In such a gorgeous palace!

In this short monologue, Juliet uses about 10 oxymorons in order to express her sorrow, disbelief, disdain, and love. For example, she recognizes that Romeo's good looks may have caused her to overlook his evil heart: "O serpent heart, hid with a flowering face!" Furthermore, the word "tyrant" has a negative connotation, yet she pairs it with the word "beautiful," just as the word "fiend" has a negative connotation, yet she pairs it with the word "angelical." Throughout this soliloquy, her emotions for Romeo are quite conflicted, which could bring us closer to your thesis.

When a person is infatuated with someone, he or she will experience an intense passion for the person; however, that passion is normally short-lived. 

In the case of Juliet, we can argue that her passion for Romeo was intense due to the fact that they married so quickly. We could also argue that her passion for Romeo is intense using the diction in this very monologue. What forces that infatuation to be short-lived, however, is the fact that Romeo killed her cousin Tybalt, thus, making Juliet feel conflicted between honoring her new husband or her family. 

Here are some basic and more advanced thesis statements you could work with:

In the monologue 'O serpent heart, hid with a flowering face,' William Shakespeare highlights Juliet's infatuation with Romeo through oxymoronic phrases. 

In the monologue 'O serpent heart, hid with a flowering face,' William Shakespeare reveals Juliet's "true love" for Romeo as just infatuation through conflicting, oxymoronic phrases.

In the monologue 'O serpent heart, hid with a flowering face,' through the use of oxymoronic phrases, William Shakespeare shows that Juliet's infatuation with Romeo is causing her to feel conflicted between her duties as a wife and a daughter.

In this monologue, Shakespeare uses numerous oxymora to highlight Juliet's discovery that Romeo is more than just the pretty face she first encountered and that she herself has contradictory feelings about the nature of their relationship.

Juliet has just learned that Romeo killed her cousin Tybalt in a street fight. She is horrified to learn that the boy she has fallen for is capable of such violence and bemoans her discovery. It seems Juliet must wrestle with the fact that Romeo is not simply a sweet young boy. Juliet fell for Romeo quickly, and on such shallow grounds as his appearance, which her monologue suggests she is coming to realize. Juliet obviously feels betrayed by Romeo's act, but she also distances herself from any responsibility by blaming "nature" for clothing "deceit...in such a gorgeous palace." In this way we can see that Juliet, while forced to re-examine her feelings for Romeo, will choose to ignore any misgivings and continue to pursue her infatuation, and in doing so keep both of them moving toward a tragic end.

First, check out the link I included. There is a nice discussion on love vs. infatuation.

The irony of using this quote is that it is the only time that anyone shows something like love. Shortly after this, Juliet decides to trust Romeo, which shows she feels something beyond the situational excitement of infatuation.

However, you have said that you are trying to connect this monologue to infatuation, so you may want to focus on her references to physical beauty. Juliet refers to Romeo as having "a flowering face" and "sweet flesh." Her positive side of nearly each metaphor is about his looks or his being divine to some degree. Infatuation often focuses on physical attraction and engulfs both parties so much that it seems they worship each other, so the heavenly references and focus on beauty could point to infatuation. 

Here are a few arguments that might help you:

1. Juliet's quick dismissal of Romeo after Tybalt's death shows that she was focused on Romeo's external beauty and was not truly in love with him.

2. Juliet focuses on Romeo's physical features rather than expressing any dismay about his actions, so their attraction is more akin to infatuation than love. 

3. When Juliet hears that Romeo killed her cousin, Tybalt, she does not defend him; nor does she seem surprised by his actions. Rather, she immediately condemns Romeo and expresses remorse that his beauty is not matched by his heart.

4. Juliet reveals an infatuation with Romeo rather than love when she denounces Romeo rather than defending him, focusing her condemnation on his character not matching his physical beauty, a condemnation that shows how quickly she thinks the worst of her new husband and how little she thought of his character before marrying him.