Horcoff's goal was the only tally for either side in the shootout. Mathieu Garon turned away David Perron, Brad Boyes and Doug Weight for Edmonton.
A penalty for too many men on the ice 5:40 into the opening period gave St. Louis the game's first power play.
59 seconds into that man advantage, Lee Stempniak found David Backes at the right faceoff circle and he made no mistake, wristing in short side past Garon to give the Blues an early lead.
Edmonton almost tied it a minute later as first Joni Pitkanen saw an opening but could not get enough wood on the shot. Moments afterward, Toivonen made a great toe save on Raffi Torres.
With five minutes left in the period, Kyle Brodziak flicked a shot from the bottom of the left circle that beat Toivonen but rang off the crossbar.
Several minutes later, Eric Brewer tried to clear the puck out of his own zone but Ales Hemsky managed to keep the puck in. He spun around and fed it to Shawn Horcoff in the high slot. Horcoff had a look and then wristed it high blocker side to tie the game at 1-1.
Edmonton's penalty kill did well in the final three minutes of the period, killing off a penalty to Dustin Penner and then the first minute of an Ales Hemsky minor to end the opening 20 minutes.
After one period, shots were 8-4 for the Oilers.
Just as the penalty to Hemsky expired, Paul Kariya found Doug Weight at the right circle for a quick shot. Garon couldn't get across in time and St. Louis took the lead at 2-1.
But 25 seconds later, Dustin Penner scored his eighth of the season to tie the game once again. Hemsky danced into the St. Louis zone but lost control of the puck as he tried to get around the Blues defenceman. However, Horcoff picked up the puck behind the net and threw it in front to Penner. Penner's shot beat Toivonen low to make it a 2-2 game.
The Blues regained their one-goal lead seven minutes into the period. Brad Boyes had the puck alone in the slot and backhanded it over Garon.
21 seconds after the go-ahead goal, Edmonton had a power play opportunity when Jamal Mayers was sent off for interference. Edmonton was unable to generate any offence on the five-on-four but had another chance several minutes later after a penalty was called on Lee Stempniak.
Late in that power play, Jay McKee was sent off for boarding, putting Edmonton on a two-man advantage.
It only took Sheldon Souray eight seconds on the five-on-three to score his first goal since returning to the lineup. Souray took a pass from Pitkanen and fired a slap shot glove side past Toivonen.
St. Louis moved back in front with Doug Weight's second of the night at 16:03 of the middle frame. Smid tried to clear the zone but it was intercepted by Boyes. He fed it over to Erik Johnson whose slap shot was tipped by Weight in front of the net.
Shots after two periods were 18-13 for Edmonton.
A Matt Walker double minor for high sticking gave the Oilers an extended power play to start the third period but 46 seconds into it, Sheldon Souray received a double minor for roughing to negate that man advantage.
After the penalties, Boyes almost scored his second of the night. He was in alone behind the Oilers defence and tried to get the puck around Garon but the Oilers goaltender stretched out his legs to make a great save.
About a minute later, the Oilers looked to have tied the game when Pisani and Cogliano jammed the net but after a video review it was ruled the puck never crossed the line.
Edmonton went on its ninth man advantage of the night when Eric Brewer was sent off for holding at 14:54. On that power play, the Oilers scored to tie the game at 4-4.
Robert Nilsson found Pisani in the slot and he roofed it on Toivonen for the equalizer.
With 2:18 to play in the third, Torres and Stoll both received penalties to give St. Louis a two-man advantage.
On the ensuing power play, Edmonton's penalty killers stepped up in a huge way. Garon made a brilliant save on Keith Tkachuk at the top of the crease.
After regulation, shots on goal were 28-22 in favour of Edmonton.
Edmonton had some good chances to win the game in overtime. Tom Gilbert skated around Barrett Jackman and skated in on a two-on-one with Gagner. His set-up pass to Gagner at the right side of the net just missed the mark.
Moments later, Shawn Horcoff broke in alone on Toivonen and tried a backhand deke but was denied.
NOTES: Horcoff extended his scoring streak to eight games with a goal and an assist. Nilsson extended his streak to seven games with his assist... The Blues had only allowed two power play goals against at home all season before Edmonton's two tonight. The last one was on October 27 versus Washington.