PORTLAND — Going into the Class B South girls basketball final Saturday, Oceanside’s closest game was a 17-point win early in the season. The Mariners had not been tested. If they were in a close game, how would they respond?

Oceanside can now consider the question asked and answered. The Mariners held off a late York rally to emerge with a 56-49 victory at the Portland Expo, earning the first regional championship in team history. Neither of Oceanside’s predecessors, Rockland or Georges Valley high school, won a regional title before merging a decade ago.

“Going into games like this, you’re not going to win by 20. That’s just not how a team gets here. When it gets close, that’s normal. You’ve just got to keep your composure,” said Oceanside’s 6-foot-1 freshman, Bailey Breen, who had 19 points, 20 rebounds and six assists.

Second-seeded Oceanside (19-0) will face Hermon (19-2) in the Class B state championship game Friday at Cross Insurance Center in Bangor. No. 5 York, which upset top-seeded Wells to advance to the regional final, ends the season at 12-6.

“We’ve worked so hard for this as a team for as long as I can remember,” said Oceanside’s Audrey Mackie, who scored 19 points and won the Mike DiRenzo Award as the tournament’s most outstanding player. “That’s probably the biggest challenge we’ve faced this year. York had really great pressure on us. We had success against it, but it also frazzled us at times, too.”

Early on, the Mariners had no problem with York’s full-court pressure, using Breen at the top of the press because of her size and ability to see the court. Oceanside led 17-6 after one quarter, and the pace of play did not suit York Coach Jess Stacy.

“We fell into more of that slow, methodical pace. We talked about it before the game, we couldn’t get into that slow, methodical pace with them. We had to run our game,” she said.

Midway through the third quarter, Oceanside pushed its lead to 38-21 on a basket by Abby Waterman, who finished with 14 points. From there, the Wildcats began their comeback. A 10-0 = run cut the deficit to 38-31 before Mackie hit a 3-pointer to give Oceanside a 41-31 lead going to the fourth.

“I thought we still had a chance,” Stacy said. “I’m one of those people, I will never count these girls out, because I’ve seen them do some pretty incredible things. Seventeen is a big number, but we started chipping away.”

York forced 10 Oceanside turnovers in the second half and pulled within two points, 51-49, on an Emily Rainforth layup with 1:18 to play. Rainforth scored 13 of her 29 points in the fourth quarter, including a trio of 3-pointers. Ava Giacobba added 12 points for York.

Down 52-49 with 39 seconds left, a York turnover effectively sealed Oceanside’s win. The Mariners went 5 for 6 at the line in the final minute, with Mackie going 4 for 4.

“I try and stay as composed as possible, because I know we can get right back and get into our sets. I think we stayed pretty composed,” Mackie said. “I think to myself, I’ve practiced these my whole life. It’s no different. Just got to knock them down.”