If you save, over the years, all the cleaned-and-dried-out holy oil/water bottles from church, before long, you'll have quite a collection of empty bottles on your hands.
If the church wants the empty bottles back after you clean them out properly and reverentially, fine. If not, clean them out with soap and water, dispose of the water in a garden where it won't be trampled on, and once you are confident that the bottle is clean and free of residue, what else can you do? If you don't want to keep it, throw it away. At that point, it's just an empty bottle. It's the content that is holy. This reminds me what Christ said in the Gospel - Which is greater, the gift or the altar that makes the gift sacred?
Please don't turn holy oil and holy water into magic potiions. No one is telling you to desecrate holy things here, and I don't think anyone is implying that. Use it for what it's intended for, dispose of the remains reverentially, and deal with the plastic bottle appropriately. Don't make a big deal out of it. Just focus on Christ; you're intention is not to defile a plastic bottle because it once contained something holy that is now all gone, but simply to dispose of it. What are you going to do, have a box in your basement with 50 or 60 washed out plastic bottles? Are you kidding me?
Goodness, we Orthodox take ourselves too seriously sometimes.