Carlisle were just a goal down when they lost skipper Lee Miller to a red card on the half-hour mark and they never really recovered from that upset.
O's, who led 2-1 at half-time, went on to win convincingly and have now scored a total of 13 goals against Abbott's men in their last three meetings.
Slade said: "I feel for Greg, who is a good friend of mine, because with all the preparation you do you can't legislate for that first half and Miller getting sent off.
"We were at the top of our game, but I'm sure Carlisle will be fine. Things conspired against them and I'm just happy to get that first win under our belts. We have taken the first step, but there's still a lot of work to be done and I thought we showed our adaptability."
There was concern about the condition of O's skipper Nathan Clarke, carried off after the clash which led to Miller's sending-off. Clarke hit his head on the concrete surrounds as he fell near the corner flag.
"He's in hospital and not in good shape. He had concussion and there was bleeding. I've watched the replay and Miller's challenge was inconclusive, but the linesman was right on top of it and the referee had a good view of it," Slade added.
"We were ruthless in front of goal in the second period in particular. I think that's my first opening-day win in 10 seasons."
- Sports & Recreation