Cut pork roast in half. Sprinkle with salt and pepper. In large skillet, brown roast in oil on all sides; drain. Place in 5 quart slow cooker.
In large bowl, combine pineapple juice, pineapple, brown sugar, celery, vinegar, and soy sauce. Pour over the roast.
Cover and cook on low for 3 to 3 1/2 hours or until meat thermometer reads 160.
Remove roast and keep warm. Let stand for 10 minutes before slicing. Meanwhile, strain cooking juices; transfer to large saucepan. Combine cornstarch and water until smooth; stir into cooking juices. Bring to a boil, cook and stir for 2 minutes or until thickened. Serve over pork.
NOTE: I like to use 2 forks and shred the pork roast instead of slicing it and return it to the cooking juices. I then serve it over rice. I also usually add the cornstarch & water mixture to the cooking juices in the crockpot instead of using a separate pan. I also sometimes add some extra pineapple chunks towards the end because my family really loves pineapple.