''Zoro's attack was stronger than Oden's''
Since some delusional people here have no eyes, let's see who made a bigger scar...
That is the Scar Oden gave Kaidou.
Since he gave him this scar when Kaidou was in his full Zoan Form - which is a lot bigger - the size of the scar is actualy bigger when Oden cut him - it's even bigger than Oden himself. It shrinks when Kaidou goes in his Human or Hybrid form, obviously.
Kaidou could not withstand the damage and pain and crash landed.
Now let's look at the scar AFTER Zoro cut him.
Now the Scar at the top right corner goes slightly into his Tatoo, if we compare those 2 pics with the scar how it was before Zoro's cut - Look first picture - and how it is now, Zoro's scar is only that big:
RED - Oden
BLUE - Zoro
As you can see Zoro made only this tiny little new scar. Oden's scar itself is many many times bigger than Zoro's and don't forget that Oden cut Kaidou in his Full Zoan form and the scar when Oden cut Kaidou was actualy alot bigger than that.
On top of that, the damage and pain caused by Zoro's cut wasn't even close to Oden's - Zoro couldn't even knock Kaidou off his feat, as stated by Zoro himself. He hoped to ATLEAST accomplish that lol
But as you can also see, Zoro cut more that this tiny new scar, but the rest of the cut was above Oden's old scar. So he cut over an scar that was already there and as Kaidou himself said to the RS, ''you can't
EVEN open the old scar''. Making a new scar is harder than opening an old one - which should be ovious to everyone here, but i wanted to mention it specificaly for some users here.
And even if we include the part that Zoro cut over Oden's old scar, it is stil smaller than Oden's Scar.
Conclusion:
Oden made the bigger scar and based on how Kaidou took the attacks - Oden's attack is clearly stronger than Zoro's.