WASHINGTON (AP) _ House Speaker Thomas P. O'Neill., performing his last ceremonial duty, helped his granddaughters push a button Wednesday night that transformed a 55- foot Christmas tree from California's Klamath National Forest into a blazing holiday sentinel on the Capitol's north lawn.

It was the 23rd annual Capitol Christmas tree lighting ceremony, a half- hour pageant of Christmas music and speeches that O'Neill has presided over for the past 12 years, 10 of them as speaker.

The Massachusetts Democrat said he returns to Boston on Thursday, with the expectation that the tree-lighting chore will be ''my last official duty.''

The 55-foot tree reached the Capitol lawn after a 3,500-mile trek from the Klamath forest near Yreka in Siskiyou County. A loyal band of local citizens were on hand to see their gleaming tree contest a slice of the Washington skyline with the Washington Monument, a mile down the mall.

Each of them wore a white cap that proundly bore the words, ''That's My Tree.''

Also from California, to serenade the tree, were the Davis Senior High School Madrigal Choir from Davis, Calif., near Sacramento.

When the singing ended, O'Neill and his granddaughters - Caitlin O'Neill, 9; Abby O'Neill, 8; and Michla Daniel, 5 - walked toward the button, intertwined their hands and pushed it.

Then came the evening's only disaster. O'Neill, moving toward the madrigal singers to say thank you and shake hands, snagged a wire with his foot and overturned the podium.

But the tree had been lit.