O'Reilly called the match off just before 12.20pm after inspecting the waterlogged Tannadice surface. The timing of the decision angered Hearts, who will today lodge a complaint with the SPL on behalf of their supporters due to the lateness of the call.
Some Hearts fans were already in Dundee when the game was postponed, but O'Reilly has outlined the sequence of events leading to the decision and said he could not have reached the stadium any earlier.
"The guys at the SFA contacted me just after quarter past ten with instructions from the club that they needed an inspection," explained the official. "I really don't know if it could have been called off any earlier. I'm travelling up from Glasgow so that was the earliest I could have got there and I called the game off.
"The primary concern for the referee is the safety of the players. The pitch would have been dangerous and it wasn't going to get any better. The easy decision to make was to put the game off. The pitch was waterlogged. Large areas of the field of play had water lying on top of it."
In a statement, Hearts' managing director David Southern said: "Our concern centres around the fact that the weather in Dundee was extremely poor overnight. We would have welcomed a much earlier decision to try and avoid this outcome."
Meanwhile, the Hearts captain Marius Zaliukas has ensured a Tynecastle double by winning the Clydesdale Bank Player of the Month award for December. He joins Young Player of the Month, Hearts' David Templeton.