Oak extended its exchange offer to March 22.

The troubled, San Diego-based cable-TV equipment manufacturer said that, as of March 13, about 79% of the three debenture issues--totaling $230 million--had been tendered in the offer, which began Feb. 8. The offer already had been extended from Feb. 28 until March 13. Oak Industries said it could save up to $28 million annually in interest payments under the debt restructuring.