This isn't specifically for mead, but in general you will get much better flavor from fruit additions if you want until after active fermentation is complete and the mead/beer or whatever is starting to clear. The amount of time you want it to be in contact with the fruit varies with the type of fruit, but in general a good 6-8 weeks will allow for good flavor extraction. Fruits with high tannin content (grapes, apricots) should get racked off fruit closer to the 6-week mark to avoid it becoming too astringent, but with any fruit if you wait too long you will start losing some of the fresh fruit character. My rule of thumb is to wait 6-8 weeks before you expect to package before adding fruit, and rack it off the fruit right into your bottling bucket or keg. Depending on what you're making, this could be right at the end of active fermentation or a year or so later.
As far as the oak goes, I would boil it a couple of times to extract as much of the harsher flavors before adding it to your mead. Err on the low side, too - you can always add more later. Same time frame for oak, I like to give it at least 6 weeks.