How do you lock spindle on Delta Osillating sander to change sand rolls?

you can google your question or better
delta boss spindle sander arbor bolt
About 565,000 result (0.59 seconds)

The link below includes the schematic, part list, pricing and ordering info for toolpartsdirect.com
Delta also offers this model in 2 other variations, 31-780 type 1 and 31-780 type-2. Both of this are also available from the page this link will take you to. The part you are looking for is $13.33 and they show 4 in stock.
Have a great day.

For the BOSS Delta Oscillating Sander
I had a really hard time removing the drum from my oscillating sander! There was no help that I could find online. So if you are having the same problem, check this out!
1. Lie the sander on its side.
2. Locate the black plastic hood. You need to remove this with a screwdriver.

3. Your sander should have come with a hex key. You need it now!
4. Put the hex key in the appropriate hole and use pliers to rotate the nut. The nut turns clockwise to loosen. This is the opposite of the way thing usually loosen!

After time spent with penetrating oil and brute strength, this was a welcome relief!

If it's the kind with a round disk sander and a wide belt sander, as far as I know belts and disks are still available. There is not much else to wear out. Make sure you test it before purchase.

Mine was underpowered from the get go. I have since sold it. Keeping it well lubricated does help though.

i used an impact tool, loosened in seconds

You use a 1/2 inch socket wrench to remove the nut at the top of the spindle shaft, slide the old sanding sleeve off and the new one on.  Replace the nut.  You may also have to insert a different size table insert if you are changing the sleeve size.
Instruction manual is available free at: http://www.dewaltservicenet.com/documents/English/Instruction%20Manual/Delta/a20932,sa350k.pdf

Remove the spindle from the machine itself before you change the sanding sleeve. There should be a flat section on the bottom of each spindle that you can hook a wrench on to hold it while you undo the top bolt.

You need to remove the allen head cap screw in the center of the base, then remove the washer, and pull the base straight down off the sander. If you can't get it to pull off, try gently rocking it a little as you exert pressure to pull it off.

Random orbit sanding base
With this sanding base, you can use the tool as a random orbit sander.
Fitting and removing the sanding base (fig. B)
- Place the sanding base (3) onto the spindle (10).
- Place the washer (7) onto the spindle.
- Insert the Allen screw (8) into the screw hole in the spindle.
- Tighten the Allen screw using the Allen key (9) supplied.
- To remove the sanding base, remove the Allen screw (8) and washer (7).
Fitting sanding sheets (fig. C)
- Hold the tool with the sanding base (3) facing upwards.
- Place the sanding sheet (11) onto the sanding base (3). The holes in the sheet do not need
to line up with the holes in the base.
Large detail sanding base
With this sanding base, you can use the tool as a detail sander or an orbital sander.
Fitting and removing the sanding base (fig. D & E)
For detail sanding, the pointed end should face forward as in fig. D. For sanding large areas,
the pointed end should face to the back as in fig. E.
- Place the sanding base (12) onto the spindle (10). Make sure that the legs of the base
locate into the tool.
- Place the washer (7) onto the spindle.
- Insert the Allen screw (8) into the screw hole in the spindle.
- Tighten the Allen screw using the Allen key (9) supplied.

You can also view the factory manual here: http://www.dewaltservicenet.com/documents/English/Instruction%20Manual/479727-00,MS1000%20SANDER.pdf

Be patient, and good luck! Let me know if this doesn't solve your problem and you still need help!

OK you will have to contact a Dewalt Service Center, They do not have that manual online at this time. If there is still a problem and you cant locate let me know and I will try something else. I found a breakdown at Ereplacementparts.com but no user manual