Celtic "have found a resilience and a will to keep going", said manager Martin O'Neill after their dramatic Scottish Cup win over Dundee.

Ethan Hamilton's outstanding strike early in the second half stunned Celtic Park, the midfielder lashing a shot past Viljami Sinisalo.

Debutant Adamu's backheel finish from fellow substitute Sebastian Tounekti's low cross in the seventh minute of stoppage time forced extra time, and Tounekti put Celtic in front just after the restart.

"The players found it, the players found the inspiration, the drive, the determination to go and actually just play to the very, very final whistle," said O'Neill.

"The players have found a resilience and a will to keep going."

After new signing Alex Oxlade-Chamberlain was introduced to the home fans prior to kick-off, Celtic's Reo Hatate and Kieran Tierney tested Dundee goalkeeper Jon McCracken from long range and Ryan Astley should have given the visitors the lead at a corner.

Luke Graham headed towards goal and Astley tried to convert but his unconvincing nod was flicked out by defender Julian Araujo.

VAR ruled Graham's push on the back of Tomas Cvancara to have started outside the Dundee box, hence a free-kick rather than a penalty was awarded.

After Hamilton struck from the inside right channel, Yang Hyun-jun touched wide for Celtic and Tierney's drive was well saved by McCracken.

Tony Yogane had an excellent chance to wrap up victory for Dundee, his shot coming off a team-mate before Celtic were able to tidy up.

Five minutes were added at the end of the 90 but a head knock to Tierney lengthened that as Celtic went right to the end to get their leveller and maintained that intensity in extra time, showing the resolve typical of O'Neill's first spell as Celtic boss.

Celtic Park had noticeable gaps of empty seats, with some supporter groups having planned a boycott in protest against the board's stewardship of the club in recent times.

O'Neill had been encouraged by the prospect of talks between fans and the club, while he spoke immediately before the match about a need to freshen his team up.

He made five changes to the starting XI and six substitutions during regulation time.

It took until Tounekti's 70th-minute arrival for Celtic to really find their stride and on-loan Freiburg forward Adamu's goal was one of the few occasions the hosts threatened in the six-yard box.

Tounekti's winner, a low finish after a one-two with sub Luke McCowan, deflated Dundee and they scarcely looked likely to get the equaliser that would have resulted in penalties.

That said, Steven Pressley's side did little wrong until Adamu's goal and will count themselves unfortunate to be out of the cup after such a sizeable effort.

For O'Neill, a three-pronged focus remains with defending champions Celtic chasing leaders Hearts in pursuit of the Premiership title and a Europa League tie with Stuttgart on the horizon as well as continued involvement in the Scottish Cup, a trophy the veteran boss won three times between 2001 and 2005.

Celtic manager Martin O'Neill: "I was thinking that we were out of the competition. Seb Tounekti's introduction helped changed things. Seb came on and gave us a brightness, he gave us something to go at, which was great and he helped make the goal. The recruit [Junior Adamu] scores a goal for us.

"In the first half of extra time, we scored pretty early on and we put ourselves in a great position."

Dundee manager Steven Pressley: "We gave so much. We left nothing out there. If we'd manage to see out one more attack, I think it would've been a very well deserved victory.

"Obviously, there was the five minutes we'd already seen out. We were a minute and a half into further stoppage time and we thought we were there. I thought we'd got there, I really did. I don't have a quibble, I'm just extremely disappointed."

Celtic host Livingston on Wednesday in a Scottish Premiership fixture (19:45 GMT).

Dundee visit Falkirk, also on Wednesday at 19:45.