The main, defining quality that attracts Winston to Julia and to O'Brien is his suspicion that they too, like himself, hate the Party. He senses, based on looks from O'Brien, that O'Brien too hates the Party but cannot speak openly about that. For this reason alone is Winston attracted to him; Winston is desperate for validation of his own feelings of discontent, and miserable in his life. If there is even one other person that feels the same way, then that is a bit of hope for him.
This same thing attracts him to Julia; she is obviously a rebel against the party, and striving to make connections with people who feel the same way. It is her most attractive feature, even though she is an attractive, young woman. He is most attracted to the fact that she hates the Party, and is willing to rebel against it. In fact, when he learns that she has been with multiple men in her life, this doesn't disgust or anger him, it makes her more attractive, because, as he puts it,
"anything that hinted at corruption always filled him with a wild hope."
Julia is also smart, well-versed in her opinions on the Party, and very coherent and logical in dissecting the Party's motives. He learns a lot from her, and that is attractive too.
The driving force behind Winston's attraction to these two key characters is their united hatred for the Party, and their desire to live outside its restrictive bonds. I hope that helped; good luck!