The Hammers looked as if they were heading for all three points at White Hart Lane until Harry Kane scored two goals in two minutes to help Tottenham come away with all three points.
Obiang says the players need to learn from their mistakes and ensure they come back even stronger at Old Trafford next Sunday.
Obiang said: “We were very upset in the dressing room after the final whistle. It was a great game and we tried everything to win but in the final few minutes we lost the points.
“Normally in football if you don't see the game out it is normal that the other team will try to come back and we need to improve and not make little mistakes.
“We need to be positive because we had played well but in recent games we have lost games in the second half and we need to change something.”
On a positive note, Obiang says he was thrilled to see Diafra Sakho play his first game of the season and hopes he will grow even stronger in the coming weeks and months.
With the Hammers now preparing for another big game against Manchester United, Obiang says the players will work even harder in training to put things right.
Obiang added: “We have another big game next week against Manchester United and we know it will be another tough game.
“If we had won against Tottenham it would have made things easier for us going into that game, but we have to keep going.
“We have to take the positives out of the game. It was good to see Sakho and Andre (Ayew) back in action and Darren (Randolph) made some good saves so we have to stay positive.”