* Boeing biggest drag on Dow
* Crude oil prices continue to slide
* Investors prepare for "quadruple witching"
* Red Hat up after FY rev forecast increase
* Indexes down: Dow 0.79 pct, S&P 0.61 pct, Nasdaq 0.50 pct (Updates to open)
By Abhiram Nandakumar
Dec 18 (Reuters) - U.S. stock indexes were lower in early morning trading on Friday as crude oil prices headed for their third straight weekly loss on global oversupply concerns.
Trading was also set to be more volatile than usual on "quadruple-witching" Friday, when options on stocks and indexes as well as futures on indexes and single stocks expire.
"Everyone is worried about the $1.2 trillion options expiration that takes place today and you're going to see some jimmying and jammying around that," said Jeffrey Saut, chief investment strategist at Raymond James Financial in Florida.
"But, the historical precedent of the December expiration is that it's usually pretty damn bullish," he added.
Saut said he expects the indexes to recoup its Thursday losses and rally towards all-time highs through Friday.
All three indexes were set for their best weekly performance in a month.
At 9:36 a.m. ET, the Dow Jones industrial average was down 137.59 points, or 0.79 percent, at 17,358.25, the S&P 500 was down 12.46 points, or 0.61 percent, at 2,029.43 and the Nasdaq Composite index was down 25.04 points, or 0.5 percent, at 4,977.51.
All 10 major S&P sectors were lower, led by the industrials sector's 0.98 percent fall. Boeing weighed the most on the sector.
Exxon and Chevron shares were down less than 1 percent.
Global markets fell on Friday, with investors turning wary about the impact of a stronger dollar and weakening commodity prices on the global economy. U.S. stocks closed lower on Thursday, led by declines in energy and materials shares.
The dollar index, however, shed some of its post-Federal Reserve rate hike gains on Friday, after the Bank of Japan reorganized its stimulus program.
Merck shares were down 1.5 percent at $51.84 after it said a European Commission decision on one of its key drugs was delayed.
Carnival Corp shares were up 2 percent at $51.67, ahead of its quarterly results before the bell.
Red Hat was up 5 percent at $82.81 after it raised its full-year revenue forecast for a second time.
Darden Restaurants was up 2 percent at $59.55 after its quarterly results.
Richmond Fed President Jeffrey Lacker is scheduled to speak at 12:30 p.m. ET.
Declining issues outnumbered advancing ones on the NYSE by 1,719 to 869. On the Nasdaq, 1,487 issues fell and 760 advanced.
The S&P 500 index showed one new 52-week high and 16 new lows, while the Nasdaq recorded three new highs and 49 new lows. (Reporting by Abhiram Nandakumar and Aastha Agnihotri in Bengaluru; Editing by Don Sebastian)