'''Theresa Diaz''' è un personaggio della serie O.C. interpretata da Navi Rawat.Lei è la figlia di Eva Diaz e la sorella di Arturo Diaz, che è il migliore amico di Trey Atwood.

== 1a stagione ==

Theresa è un’amica d’infanzia di Ryan,hanno avuto una storia d’amore con alti e bassi,ma dopo la partenza di Ryan da Chino non si sono più sentiti.
Si rincontrano alla festa del Ringraziamento,Ryan brevemente tornato a Chino a pagare debiti Trey,su sua richiesta.Si sente subito dell’imbarazzo tra di loro.
Theresa trova lavoro a Newport e consola Ryan sconfortato per la faccenda successa con Oliver.
I due hanno una breve relazione,vanno a letto insieme ma lei ritorna a Chino dal suo ragazzo Eddie che ha fatto una scenata durante una festa dei Cohen.
Eddie picchia Theresa e lei chiede consoglio a Sandy Cohen su come risolvere la situazione,Ryan lo scopre e le chiede di restare dai Cohen.
Theresa scopre di essere incinta e in un primo momento decide di abortire poi torna a Chino dalla madre portando Ryan(possibile padre del bambino)con se.

==2a stagione ==

Durante il soggiorno a Portland Theresa chiama Ryan mentendogli e dicendo che ha avuto un aborto spontaneo,convincendo Ryan a tornare dai Cohen.
Riappare in un altro episodio convincendo Ryan a credere a Marissa e non al fratello Trey

{{Infobox character | colour = #ffaa44
| name = Theresa Diaz
|series=The O.C.
| other_names =
| image = <!-- Deleted image removed: 200px -->
| caption = 
| portrayer = Navi Rawat
| creator = Josh Schwartz
| species = 
| gender = Female
| first = The Homecoming (episode 1.11)
| last =  The Man of the Year (episode 3.24)
| cause = To avoid Ryan's violent behaviour
| age = 19 (last appearance)
| occupation = Hotel-related
| title =
| episode = 13
| family = Eva Diaz (mother)<br>Arturo Diaz (brother)
| spouse = Eddie (ex-fiancé) <br>Ryan Atwood<br> (ex-boyfriend)
| children = Daniel Diaz
| relatives =
| footnotes = 
}}

'''Theresa Diaz''' is a FOX series ''The O.C.'', played by Navi Rawat. She is the child of Eva Diaz and the sister of Arturo Diaz, who is the best friend of Trey Atwood.

==Character's background==
===Season 1===
Theresa is a childhood friend of Chino. Over the Thanksgiving holiday, Ryan briefly returned to Chino to pay off Trey's debts, at the request of a still incarcerated Trey. Ryan went to Theresa's house to pick up a car from her brother that would later be used to pay these debts. During this time, Theresa and Ryan each exchanged apologies about the lack of communication during Ryan's absence, most notably Ryan's failure to inform Theresa of his exodus from Chino.

Soon after Theresa and Ryan met again, when Theresa began working in Newport for (as yet) unexplained reasons. Eventually, Theresa revealed that she and Eddie (a friend of Trey's from Chino) had become engaged. Soon after the engagement, Theresa developed cold feet (most likely due to Eddie's abusive tendencies). A short time before this, Marissa Cooper and Ryan had broken up over recent complications involving Oliver Trask. Naturally, Theresa and Ryan began a relationship soon after. Eddie persisted in his attempts to win Theresa back, even showing up at Harbor High School to confront Ryan about his suspected relationship with Theresa. This finally culminated in Eddie making an appearance at the congratulatory party for Caleb Nichol's being named Man of the Year. Eddie and Ryan came to blows, with Eddie being dragged away from the party and Ryan drenched in the pool. Theresa realized that she needed to resolve these issues, and left Newport to return to Chino and Eddie. 