Hi Letitb
You don't say what condition it is in.
If the door is painted or varnished then use a hot air gun on medium heat and a scraper to remove the paint or varnish.
Once the paint or varnish has been removed then apply linseed oil with a paint brush. Paint on as much oil as the wood will absorb. You can buy more expensive wood oils but they are mostly based on linseed oil so you have no need to go to extra expense.
I give my wood a wipe over with the linseed oil about every three months. This produces a really lovely glow.
The treatment works for all wood.
Good luck and best wishes.
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antique doors