Fox’s one-time hit teen drama “The O.C.” returned with a thud last night, posting the show’s worst-ever rating and raising serious questions about the network’s Thursday ailing night schedule.

“The O.C.” averaged a 1.5 adults 18-49 rating in its fourth-season premiere, according to Nielsen overnights, placing fifth in its timeslot. The show was down by more than half from a 3.3 for last year’s third-season premiere and struggled just as mightily among viewers 18-34, where the show averaged a 1.7 rating to last year’s 4.4.

Without question, “O.C.” faced much tougher competition this year in its premiere than last year, when Fox debuted the show in early September, before the baseball break.

Last night “O.C.” returned for the first time in nearly six months facing not only CBS’s “CSI” but also ABC’s “Grey’s Anatomy,” the top-rated show among 18-49s, which moved to Thursdays in September.

That no doubt led to part of the major ratings falloff for the show, which has been fading for quite some time. It’s also possible the death of Mischa Barton’s character last spring sparked some angry fans to stay away.

But “O.C.’s” lead-ins gave it no help whatsoever. The return of “’Til Death” and “Happy Hour” combined to average just a 1.6 at 8 p.m., pushing Fox all the way down to sixth place for the night.

For two years, “The O.C.” has been the only Fox show to draw a decent audience on Thursdays. Now the network may have to consider some major schedule switcheroos to patch up the ailing night.

Meanwhile, ABC continued its Thursday winning streak, averaging a 5.6 rating and 14 share last night among adults 18-49, a tad higher than CBS’s 5.3/13. NBC was third with a 4.8/12, followed by Univision at 2.6/6, the CW at 1.8, and Fox at 1.5.

At 8 p.m., CBS’s “Survivor” led with a 5.5, followed by ABC’s “Ugly Betty” at 4.4. Among households “Betty” edged “Survivor” with a 9.3 to a 9.0.

As for the rest in 18-49s, NBC’s “My Name is Earl” (4.1) and “The Office” (4.2) combined for a 4.2, followed by Univision’s powerful “Latin Grammys” at 2.6, the CW at 2.1 for “Smallville” and Fox at 1.6 for “’Til Death” (1.7) and “Happy Hour” (1.4).

At 9 p.m., ABC’s “Grey’s Anatomy” led with a 9.0, followed by CBS’s “CSI” at 6.9, NBC’s “Deal or No Deal” at 4.6, Univision’s “Grammys” at 2.7, “The O.C.” at 1.5, and the CW’s “Supernatural” at 1.4.

At 10 p.m., NBC slipped into the lead with a 5.6 for “ER,” followed by CBS’s “Shark” at 3.7, ABC’s struggling “Six Degrees” at 3.2, and Univision’s “Grammys” at 2.5.

CBS led the night among households with a 10.2/16. ABC was second at 9.5/15, followed by NBC at 7.4/12, Univision at 3.1/5, the CW at 2.5/4 and Fox at 2.4/4.