The Federal Trade Commission yesterday said it is opposing the buyout of Brockway Inc. by Owens-Illinois Inc. because of concerns that the merger would reduce competition in the glass container industry.

The commission voted 3-2 to authorize the staff to seek a preliminary injunction to block Owens-Illinois, the nation's second-biggest maker of glass containers, from completing its $750 million acquisition of Brockway, the third-largest.

It was the second time this week that the FTC opposed a merger on competitive grounds. The agency on Monday voted 5 to 0 to oppose Dun & Bradstreet's acquisition of Information Resources Inc. Dun & Bradstreet withdrew its offer Tuesday.

Shortly after the commission announced its action yesterday, Owens-Illinois said it was meeting with the FTC staff to discuss ways of resolving the concerns. That could include the possible sale or shutdown of some of the company's production capacity, analysts said.

Owens-Illinois also extended its $60 a share tender offer for Brockway stock through midnight Nov. 24. It was to expire at midnight yesterday.

Brockway shares, already depressed following the stock market's Oct. 19 plunge, fell $5.87 to $47.12 on the New York Stock Exchange.

If Owens-Brockway merger had been approved as initially proposed, it would have put more than two-thirds of the nation's glass container-making capacity under the control of two companies.

Owens-Illinois, which is based in Toledo, Ohio, had 1986 glass-container sales of about $1.1 billion, giving it about 24 percent of the market. Brockway, based in Jacksonville, Fla., had glass container sales of about $681 million last year, accounting for 16 percent of the market.

The former No. 2 producer, Anchor Glass Container Corp., became the biggest earlier this year by acquiring Diamond-Bathurst Inc. The combined companies accounted for about 24 percent of the market last year.

H. Edward Schollmeyer, who follows the companies for the investment firm PaineWebber Inc., said there had been unconfirmed market rumors of opposition to the buyout by certain food manufacturers that rely heavily on glass packaging. However, he said he did not believe any formal complaints were filed.