Hello and welcome to the forums!
From what I can tell by reading your post, you are leaving the cells very long in serum-free medium. These OKP cells what type are they?
I would try transfection with Fugene-6 which does not need changing of media or serum-free media. your cells will definitely do better with the media in place at least while you transfect. Lipofectamine is toxic although it shouldn't be that toxic, however with 24 hours without serum that is not so good.
Try to optimize your transfection conditions, use Fugene-6 or other transfection reagents allowing serum media keeping your cells healthy!
and make sure you tell us how you are doing soon!