Why does my guitar buzz only on open strings?

Usually the nut slots are cut too low, or the first fret sits high. Press the string between the 2nd and 3rd frets: if it touches the 1st fret, the nut slot is too low.

Should I adjust the truss rod or the saddles first?

Set the neck relief with the truss rod first, then the string height at the saddles. Changing the saddles first often just moves the buzz along the neck.

Why did fret buzz start after I changed strings?

A different gauge pulls on the neck with a different tension. Give the neck a day to settle, then recheck the relief before anything else.

Fret Buzz Finder

Play each string open and up the neck, and note where the buzz is. Then answer three questions to get the likely cause and the first fix to try.

Questions

Why does my guitar buzz only on open strings?: Usually the nut slots are cut too low, or the first fret sits high. Press the string between the 2nd and 3rd frets: if it touches the 1st fret, the nut slot is too low.
Should I adjust the truss rod or the saddles first?: Set the neck relief with the truss rod first, then the string height at the saddles. Changing the saddles first often just moves the buzz along the neck.
Why did fret buzz start after I changed strings?: A different gauge pulls on the neck with a different tension. Give the neck a day to settle, then recheck the relief before anything else.

More detail

The finder follows the decision table in ourguitar fret buzz guide. For the numbers to aim for on your model, use thesetup specs lookup.