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If you mean the EF 24mm, I've never had one of those.
While I have one, I've not done any direct side-by-side comparisons with the FDn 24mm f/2.8.
They're both pretty good on film. On digital I tend to use the Oly more on the 5D and the FD more on m4/3. My gut feeling is that the FD has lower distortion than the OM, but that the OM is a little sharper.
The OM 24 is tiny, but I find Canon's lens hoods easier to live with than the screw in metal ones Olympus use, even if the Canon ones are more likely to fall off.
I'm itching to purchase a Sony A7 and put them up against each other on FF digital.