I’m looking at buying a second hand table; I’ve found the one I want (is about 40 years old) but the colour might be out by a few shades. Is it straightforward to sand off the finish, and re-stain and re-oil it with a different stain?

The answer is both yes and no. Theoretically it's very straighforwards but in practice it can be more challenging and time consuming depending on the existing timber, finish and so forth, so it can't always be guaranteed.

Ah.  40 years ago.

Black ash.

 

 

I miss the 80s.

Do you know what timber the table is made of?

40 years old could be french polished or varnish both of which can be removed easily.

if the wood is soft and has been stained then the colour is deep set and difficult to change (lighten) but can be darkened or reddened 

1 minute ago, markc said:

french polished or varnish

Or polyurethane.

38 minutes ago, markc said:

Do you know what timber the table is made of?

It might be maple. Not sure. Here is a photo:

Only just seen this, looks like polyurethane varnish. Easy to sand and will take a darker stain. Turn table upside down, sand and try the stain.