O'Brien Must SufferTM. Again. Not that I'm complaining. The demon possession in this episode actually works extremely well.
Memory Alpha has a recap here.
This is the first time we see a Pah-wraith possess one of the characters, and to be honest with you, I like this early version of the concept better than the red-eyes-and-modified-voice possessions we'll see later. Because Keiko looks and sounds entirely normal, there's a lot of opportunity here to milk the dramatic irony for all it's worth. And milk it the writers do! The fact that O'Brien must behave as if nothing's out of the ordinary - indeed, he's even forced to share a bed with his possessed wife - adds a delicious level of tension to the whole episode.
Of course, this episode also fits quite nicely into the tradition which began with In the Hands of the Prophets in the first season and continued in episodes like Destiny and Accession, namely: The writers don't treat the Bajoran faith as a local curiosity that its practitioners cling to out of mere sentimentality. Instead, the Bajoran faith is acknowledged to have some claim to the truth, and its legends are validated and incorporated into the show's larger myth arc. There was a war in heaven, the writers confirm, and demons do live in the Fire Caves. Now, a Starfleet officer is still perfectly free to interpret the battle between the Prophets and the Pah-wraiths in entirely secular terms, but that doesn't change the fact that the Bajoran tales of the Koss'moran (or, later, the Kosst'amojan) are based on something real.
Lastly, just as a side note, this is also a great episode for Rom. When the number of Ferengi in an episode reaches a certain critical mass, inanity usually results -- but when you isolate the Ferengi characters, you get stories like this one in which Rom isn't reduced to wailing "Moooooooooogie!" every five seconds because Ira Steven Behr inexplicably thinks that's hilarious. In fact, not only is Rom portrayed as adorably loyal here, but he is also allowed to put the pieces together and draw his own conclusions about the Pah-wraith's plans, thereby proving that he isn't as stupid as he sometimes looks. And I like that. I like that a lot.
The writers do a fantastic job maintaining the squick factor with Keiko's possession, and Rom's scenes are very refreshing.
The performances are strong all around, which is generally typical of an O'Brien-centric episode.
See the above discussion of the show's incorporation of Bajoran legends.
ROM: What's this?
QUARK: What do you mean, what's this? It's puree of beetle.
ROM: I didn't order it.
QUARK: Of course you didn't order it. You don't need to order it. You have it after work every morning.
ROM: Well, I'm not having it this morning. I'd like two eggs over easy, three strips of bacon and a side of corned beef hash.
QUARK: What kind of breakfast is that?
ROM: It's Chief O'Brien's breakfast of choice.
QUARK: That's the problem with working alongside humans: you pick up their disgusting habits. Bacon. Corned beef hash. What's next, Rom? Orange juice and coffee? (Heh.)
O'BRIEN: I'm sorry, sweetheart. We'll buy some new ones.
KEIKO: Forget it, Miles. They're just plants.
KEIKO: It's not as if something happened to Molly or the baby.
O'BRIEN: Right, right, right. Everybody's fine. No problems.
KEIKO: Good. What about Kira? Is she back yet?
O'BRIEN: No, she's still with Shaakar travelling around Dahkur Province. I thought you were going to visit them.
KEIKO: I didn't get a chance.
O'BRIEN: So, how were the Fire Caves?
KEIKO: Fine. And now it's time for me to tell you some news. I'm not Keiko.
O'BRIEN: No? Who are you?
KEIKO: Listen carefully, Miles. I have taken possession of your wife's body. I will hold it hostage until you do everything I tell you do accurately and without question.
KEIKO: If you don't do precisely what I ask, I'll kill your wife. (Eeeep!)
O'BRIEN: You'll have to be patient. The communications and sensor relays are distributed throughout the entire station.
KEIKO: You know your wife well, Miles, but she knows you even better. I know you're just playing for time until you can get to your friends. Julian, Dax, Captain Sisko -- I know they'll all want to help you. The Captain may even allow you to do what I'm asking. At least until someone figures out a way to trap me in some sort of stasis field or some other clever device you're already dreaming up. And you know what? It might work. You might be able to stop me. But I promise you one thing. If you do, Keiko will die. All I need is a split second to cause a massive brain hemorrhage and she's gone. (EEEEEEEEEEEP!)
KEIKO: Get a hold of yourself.
O'BRIEN: Look, I've done everything you asked me to do. Just give me back my wife.
KEIKO: You've done well, Miles. But recalibrating the optronic integrator was only a test. I had to be sure I could trust you. Tomorrow the real work begins. Now it's time to get back to our guests. We don't want to raise suspicions, do we? (Holy crap, this episode is creepy.)
KEIKO: The q'parol was a big success.
O'BRIEN: Look, no more tests, no more games. Just tell me what you want done.
KEIKO: We'll talk about it in the morning. It's late and you're working a double shift tomorrow.
O'BRIEN: I want to talk about it now!
KEIKO: Miles, I make the rules. Now relax. Everything's going to be fine as long as you do what I ask.
O'BRIEN: I'll sleep on the couch.
KEIKO: Don't be childish. What if Molly has one of her nightmares and comes running in? You'll sleep in bed with me, just like you always do. (See what I mean?)
KEIKO: You were going to tell them about me.
O'BRIEN: How did you know?
KEIKO: I know everything your wife knows and she knows you. I told you we have to trust one another. You violated that trust. Now look where it's led us.
O'BRIEN: I want my wife back, do you hear me? I want her back.
KEIKO: Then stop trying to fool me. How much more damage do you think this body can take? (Wow.)
O'BRIEN: Rom, I need your help, and I need you to keep your mouth shut. Can I count on you?
ROM: Is that a trick question?
O'BRIEN: Rom, this is serious.
ROM: You can depend on me, Chief.
O'BRIEN: We're going to make some modifications to several of the station's systems. I don't want you to breathe a word of this to anyone. That includes Dax, Odo, Captain Sisko, Worf --
ROM: They don't know?
O'BRIEN: Of course they know. At least, some of them do. But they have to pretend that they don't. This is a top secret Starfleet operation.
ROM: Culpable deniability. I understand. Don't worry about me, Chief. My lips are sealed. Nobody will get anything out of me. Not even my name.
O'BRIEN: Rom, everybody on the station knows your name.
ROM: Right. (A beat.) But I won't confirm it. (LOL!)
CREWMAN [OC]: Chief O'Brien, I have a call from your daughter.
SISKO: Go ahead.
O'BRIEN: Molly, honey, what is it?
MOLLY [on monitor]: Hi, Daddy. Mommy told me I could call you.
KEIKO [on monitor]: (brushing Molly's hair) I hope we didn't interrupt anything important.
O'BRIEN: It's all right, Molly. I'm glad you called. What are you doing home?
KEIKO [on monitor]: She missed her mommy. Isn't that sweet?
MOLLY [on monitor]: When are you going to come --? (Just then, the wraith pulls Molly's hair.) Ow! Mommy, that hurt!
KEIKO [on monitor]: Oh, I'm sorry, sweetie.
O'BRIEN: Are you all right, Molly?
KEIKO [on monitor]: She's fine, Miles. She wants to know when you'll be finished. I told her you had to work another two hours, twenty two minutes and thirteen seconds.
MOLLY [on monitor]: How long is that, Daddy?
O'BRIEN: Soon. I'll be home very soon.
KEIKO [on monitor]: Good. We both miss you. (Oh, man, the subtext is thick here.)
O'BRIEN: What is it, Rom? (Rom hesitates.) It's okay. I've disconnected Odo's eavesdropping devices.
ROM: I haven't told anyone anything, just like you said.
O'BRIEN: You're a good man. Now why did you call me? I told you we're under very tight time constraints.
ROM: Captain Sisko is really angry. If I didn't know he was pretending --
ROM: Don't worry, I can hold out until you say otherwise, but there is one thing I have to know.
O'BRIEN: Go ahead.
ROM: Why are we focusing a chroniton beam at the wormhole?
ROM: The new frequency of the deflector grid is set to turn the station into a massive chroniton array aimed directly at the wormhole. So I just wanted to know: why are we trying to kill the wormhole aliens? (Oh, snap. Rom is smart.)
ROM: It's a Bajoran legend. From the verb kosst, meaning 'to be' and amoran 'banished'. Leeta's been telling me all about Bajoran legends. She can go on for hours. She says I'm a good listener.
O'BRIEN: I'm sure you are. Now's the time to be a good talker. Go on.
ROM: Well, according to Leeta, the Pah-wraiths used to live in the wormhole. They were part of the Celestial Temple.
O'BRIEN: They were Prophets?
ROM: False Prophets. They were cast out of the Temple, exiled to the caves where they were imprisoned in crystal fire cages and forbidden to ever return lest they face the wrath of the true Prophets.
O'BRIEN: So if these false Prophets were to return to the Celestial Temple?
ROM: I don't think they'd be welcomed.
O'BRIEN: Unless she kills all the wormhole aliens first. Rom, I'm going to have to leave you in a tough spot.
ROM: Captain Sisko, Odo, they don't know about any of this, do they?
O'BRIEN: No. No, they don't. And I want you to help me keep it that way for a little while longer.
ROM: I have to stay here and play the idiot?
O'BRIEN: I'm afraid so. No matter what Odo asks you
ROM: I'm Quark's brother. I know the role. (Heh. Indeed.)