If you only wish to know the non-spoiler points of Oculus, here's the skinny: it is frightening. It is go-home-and-leave-the-lights-on-in-case-something-weird-is-in-MY-mirror scary. Through a mix of children in jeopardy, distortions of perceptions and reality, and some good old-fashioned dead people who pop out every so often to shriek at you, it hits on a ton of fear buttons. It doesn't hurt in that department that two of our favorite sci-fi heroines - Katee Sackhoff and Karen Gillan - are put in severe jeopardy of life, limb, sanity and mutilation (neither comes out completely unscathed).
And Gillan, in her first big movie lead role since leaving Doctor Who, proves more than capable of carrying a film. This is her film and she owns it, playing a character who is most definitely not Amy Pond, and not even Scottish - her American accent may not be region specifics, but it never slips.
The movie does still have issues - but the two primary reasons you're likely to go see it are not among them.
Oculus does not start with Gillan, though. It opens with a mental patient named Tim (Brenton Thwaites) being released after finally coming to terms with a memory of shooting his father. It's not a bad setup, but Thwaites is such a stiff at first that these scenes start to set off warning bells - the actor looks like he was created in a lab to be the most generically good-looking young actor there is, and his range seems to transverse only the tiniest gap between "mellow and handsome" to "slightly agitated and handsome." Mr. Thwaites is probably a very nice man, but here he seems like a generic movie star "type" rather than a real person who's disturbed by childhood traumas.
Fortunately for us - though not, as it turns out, for the character - Karen Gillan is his sister, Kaylie. She's perfectly kind and supportive, but she has one favor to call in from a childhood promise. Now out of foster care, she is inheriting the family's old stuff, but before it all gets auctioned off, she has a score to settle...with a mirror. Again, fortunately for the audience but not Tim, he's been through so much therapy that he dismisses her theory that the mirror is evil, thus requiring a whole ton of exposition about their past and the various previous owners, who are sort of like prior incarnations of the Doctor except they were all awful and died horribly. Regardless, Kaylie knows her trivia on them like a Whovian would know their Tom Baker from Colin Baker.
Knowing that the mirror's primary tactic is to alter people's perceptions of reality, she has rigged up numerous cameras, timers, lights and fail-safes in order to record what actually goes down as opposed to what her mind will perceive (a nod to Blair Witch 2, I thought while viewing, and director Mike Flanagan has since confirmed). The one mistake she makes is to only have her crazy brother as a witness. Like, in daylight, with a crowd of people around, you'd think smashing the mirror might be easy. But then there'd be no movie, so she waits until it's night, sets all this up in their childhood home where both were traumatized as youngsters, with someone whose shared memories are every bit as unreliable as hers. Maybe she's too used to the malevolent ghost always turning out to be a misunderstood alien.
Initially, we see their childhood through flashbacks, via outstanding younger actors Garrett Ryan and Annalise Basso, as their parents' marriage disintegrated. Increasingly selfish Dad (Rory Cochrane) stays locked up in his home office, chewing off his fingernails and possibly having an affair, while increasingly alienated Mom (Sackhoff) drinks wine, cries, and grows ever more resentful until visions in the mirror turn her homicidal.