Though I haven't had any experience with doing it, my feeling is that brown oil paint in the grain filler is likely to form a semi-protective layer that the stain won't penetrate properly, or at least make an uneven stain take-up more likely. I'd certainly try this out on scrap wood first before doing it on the guitar. I don't know what stains you plan on using but a spirit-based stain is more likely to work over an oil-paint base than a water-based stain. And there's obviously far less grain raising of the wood with spirit-based stains compared to water-based ones.