Opal forgets about Winn-Dixie because she's so preoccupied with saving her outdoor party from a sudden thunderstorm. She and Gloria Dump threw the party together, and the impromptu downpour threatens to destroy everything. When the rain hits, Opal snatches the egg salad sandwiches and runs them into Gloria's kitchen. Then she tries to take Gloria by the arm and lead her inside. She spots Otis by the garden, gazing at his feet, and beckons him to come inside, too.

It's not until Gertrude squawks "Dog" that Opal realizes Winn-Dixie is missing. In all the commotion, Opal simply forgot about Winn-Dixie who, like most dogs, is terrified by thunder and ran away. Though she's happy that she helped ferry the party-goers (and the food) to safety, Opal feels guilt over not protecting Winn-Dixie and holding tight to him when the storm began.

Why does Opal not realize that Winn-Dixie had run off in Because of Winn-Dixie?

Opal does not realize that Winn Dixie has run off because, in the confusion that resulted when the rain started coming down, she simply forgot about him.

Opal had been at an outdoor party she and Gloria Dump were throwing for all the neighbors.  Otis, who is somewhat of a recluse, had just arrived, and Opal had been introducing him to everyone.  Right in the middle of this, thunder suddenly rolls in the sky, and it begins to rain.  Everyone starts running around, trying to save what they can.  Opal grabs the platter of egg-salad sandwiches and runs into the kitchen with the preacher, who has grabbed the punch.  She then runs back out and sees that "Amanda had hold of Miss Franny Block and was helping her into the house", and goes to the aid of Gloria Dump, who grabs her arm as she is led to the door.  Opal looks around the garden once more before she herself goes inside and notices Otis, "standing there by his jar of pickles, looking down at his feet".  She hollers at him to come on in, and he does.

Opal is so busy worrying about the party things and making sure everyone else is all right that she completely forgets about Winn-Dixie.  The otherwise fearless dog is for some reason terrified of thunder, and Opal knows she should have held onto him when the storm hit.  Even though she had done a good job of taking care of all the other party-goers, Opal feels terrible that she forgot to look after Winn-Dixie (Chapter 23).