I had put wood conditioner on an oak piece I need to stain but I had something come up and now the piece has sat for a few weeks. Can I go ahead and stain it or do I need to repeat the process with the conditioner first?
What is sold as wood conditioner1 isn't intended for oak but now that you have used it a long wait after application isn't an issue and you can apply stain as normal.
You can however expect the colour to end up lighter than if you were staining bare wood because "wood conditioner" or anything applied for a similar purpose work by partially sealing the surface, which limits stain penetration.
As your oak has sat around for a while since you last worked on it it would be advisable to dust it well before stain application.
1 Products like this (also sold as "pre-stain conditioner" or "blotch controller"), or heavily diluted shellac, varnish or glue which can be used for the same purpose, are for reducing blotching in species prone to that problem, and oak isn't one of those species.