Hi @Fenella,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Generally speaking, no you can't apply oil-based decking oil over water-based decking oil or vice versa.
Oil-based decking oil creates a water-resistant layer, which can prevent a water-based product from adhering properly.
Water-based decking oil forms a surface film, which can make it difficult for oil-based decking oil to penetrate the timber.
If you want to transition from one to the other, you will have to sand the timber bare before recoating with your product of choice.
Check out How To Sand A Deck for some guidance on what this involves.
Let me know if you have any further questions.
Jacob
