However, I think Harry was much more annoying and Cho's reactions were much more understandable.
Let's take a look at Cho's situation at the beginning of OotP: She is a sixth-year, in Ravenclaw and on the Quidditch team. She had one boyfriend that we know of, Cedric, the Hufflepuff Kepper and one of the Triwizard Champions. Even before the end of OotP she seems to at least like Harry, she was never anything but nice to him.
Now Harry returns from the graveyard with her boyfriend's dead body at the end of the Triwizard Tournament - which was supposed to be as safe as possible. Dumbledore and Harry say Voldemort killed Cedric and Cho believes it. However, she does not hear a word from him over the holidays. Cho maybe reads the Daily Prophet which says Harry has lost his mind.
However, she doesn't believe them. When we meet her at the beginning of OotP, she is nice to Harry, supports him, just seems very confused by her feelings.
Her confusion is understandable. Here is the boy that saw her boyfriend die, the one that claims something noone else seems to believe and especially the ones for which she thinks she is developing feelings for; for which she might have had feelings even before Cedric died. This also causes guilt, as she thinks she is dishonouring Cedric.
I won't even bother here with Ron's attack on her chosen Quidditch team here, by the way. That was clearly out of line, but that was Ron's fault.
Dumbledore's Army is formed. And then comes the infamous last lesson before the Christmas break (OotP, p. 402 f., British children's edition). Harry stays behind a bit to get a 'Merry Christmas' from her. Only fair. Cho begins to cry. Somehow, she begins to think about Cedric, which is normal for someone who lost someone; you can't control when the memories come back.
By the next paragraph, we can already assume that Harry doesn't like this situation and wants to get out. He ought to have known. She wanted to talk about Cedric. He says there was no chance to survive for him and she answers that he survived. Also fair enough. But instead of saying that his mother saved him, which sho might have understood, he shrugs it off.
And he turns to go.
I think it's time for a 'What the fuck?' He supposedly loves this girl, she stands there crying and he wants to leave. The only thing I can blame her for in this situation is that she kisses him instead of slapping him. If she had wanted to be alone, she would have hid her crying.
And then this sentence: He'd have been so pleased with just a 'Merry Christmas'. Not one for taking the good and the bad in a relationship - arguments that he is still young don't apply, I think. He should know that a bit of comforting should be in order. Well, at least he feels heartless about saying he wants to forget and not talk about how Cedric died.
Cho realises his awkwardness and tries to change the topic. Harry responds mono-syllabic and really wants to get out. Then she points out the mistletoe and later they kiss. Not horribly un-romantic, IMHO. I think it did help her.
Common room. Ron says "What sort of person cries while someone is kising them." Harry's reply? "Yeah, who does?"
Excuse me. If that isn't one of the most insensitive things I have ever read this boy saying I don't know what is. And even as Hermione explains it to him, he doesn't really realise what he's been doing wrong.
Next scene in question: Valentine's Day (page 493 ff.). He doesn't get her a card or something, but well, they don't yet have a real relationship.
The date starts alright, they talk about Quidditch and so on.
Then the catastrophe begins. Harry says that he is meeting Hermione soon and whether she wants to tag along. And that it wouldn't matter to Hermione if Cho came along. Oh, how nice..., but shouldn't this be the other way around? What happened to asking your date whether it would matter to them if you met someone else with them? Okay, Harry isn't experienced with dating. Still, tact is not exactly his strength. Cho of course leashes out with sarcasm at that. Can anyone really blame her? But Harry somehow still doesn't get it.
So she tries to get him to do something. Granted, trying to make him jealous was not the best course of action.
Why she tells about Cedric next could have various reasons. It could just be that she is reminded of him because he took her there last Valentine's Day and wants to talk to someone about him. It could be because she knows Harry doesn't want to talk about him. And it could be because she was looking for something to talk about with him and that's the first thing that comes to her mind.
Harry gets annoyed with her. And he does the worst thing he could do - again tring to completly change the subject instead of comforting her. She begins to cry. Considering how she must be feeling not too surprising.
And again he does the worst: Mention Hermione. And then laughing in her face (it must look like that to her).
Cho runs out and so ends their first and last date.
And the next time we see Harry and Cho, Cho tells him she would have never though her friend would sell them out. He doesn't respond, so she tries to defend her friend. And Harry screams at her. Then she makes what I admit is a mistake: blaming Hermione.
Next time we see Cho she is dating Roger Davies.
All in all, I say Harry acts very selfish when it comes to Cho. He wants to be with her, but isn't prepared to deal with the fact that she might need someone that helps her a bit. He gives her reason to believe he likes Hermione much more than her and that he doesn't want to be with her when she is sad. So I am much more annoyed with him than with her.