Home keeper David Hay had already made a useful save from Gary Kelly's long-range effort by the time One marked his home debut by giving the Blue Brazil an early lead with the aid of a slight deflection.
Buchanan then tripped over and David McKenna was denied by keeper John Dodds while Hay parried a fierce drive from Kevin Gordon at the other end.
Midfielder Iain Mauchlen kept the home side pushing forward and after firing narrowly over from 20 yards the same player set up defender John Ward who wildly blasted over.
Ward made an even bigger mistake four minutes later when a blatant pull on the jersey of Joe Savage allowed Kelly to slot home a penalty equaliser.
Buchanan's superb angled finish quickly restored Cowdenbeath's lead and Buchanan then grabbed his 13th goal of the season from point-blank range on the stroke of half time.
The woodwork denied Buchanan his hat-trick and Dodds made a fine save from One as Cowdenbeath continued to dominate.
Only Kelly offered any resistance with a blazing half volley which flashed wide.
Mauchlen completed the scoring 18 minutes from time from another One flick and McKenna could even afford to miss a late sitter for the home team.