There are a set of circumstances around Oskar's running away that need to be considered.
- The training at the pool house would be over at around half past eight, which gives the time when Jonny and Jimmy (and Eli) arrived.
- Oskar's whereabouts between when he disappeared from the pool house and when he was sighted by Stefan Larsson the next day are unknown.
- At the train Oskar has two cardboard boxes in addition to a large trunk. The cardboard boxes presumably are Eli's boxes that Oskar had kept in his mother's storage compartment in the basement where he lived. Thus, Oskar and/or Eli must have passed by to collect the boxes.
- The trip by train from Stockholm to Karlstad takes about three hours, which sets the departure time to around noon if Oskar is to arrive at Karlstad after sunset.
Questions to answer:
Why didn't Oskar and Eli leave earlier?
Where did Oskar (and Eli) spend the night?
The fact that Oskar did not leave until fifteen hours after the pool event must have some importance. It also makes it hard to imagine a scenario where Oskar decides to run away with Eli just by an impulse. There was too much time for that. Furthermore, Oskar had already spent at least a couple of hours contemplating the pros and cons of running away with Eli, after he mistakenly believed he had become infected from handling Eli's blood-soaked clothes. He was familiar with the idea.
So, something must have held Oskar back. Something made him hesitate taking the final step. What?
Perhaps the thought of his mother. I don't know. Perhaps he didn't, perhaps he had too much else to think about. But he must have been thinking.
Or maybe he spent all those fifteen hours trying not to think at all, because all thoughts were too upsetting. So Oskar spent the hours numbly unthinking until he finally realised something that made him come into a sudden hurry. Given my idea that O&E spent the night in Oskar's basement, his revelation might have been:
They are going to search for me! They will find us here! I must take Eli away. NOW!
That way, the decision was in a way forced on Oskar. He had no alternative. But later at the train, when things had calmed down, Oskar found that he was happy with the way things turned out in the end.