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I am currently looking at a reservoir and have noticed that one well has significantly higher Sw
than the other nearby wells that penetrate the same oil bearing reservoir.

let me call the well with higher Sw, Well 'A'. and the other two wells, Well 'B' and Well 'C'.

Well 'A' has an OWC while Well 'B' and 'C' both have ODT.

Water Saturations:

Well A : 35%

Well B : 27%

Well C: 19%

My thinking is that the higher Sw in well 'A' could be due to a higher Swirr linked to the clay mineralogy in that well.
Does anybody have any ideas?

Do you see any indication of higher clay content in this well? Increase in Neutron-Density separation?

Sounds reasonable.

Have you already ruled out structural control on the water saturation? Is well C structurally the lowest? You did mention an OWC.

When looking at a gradient of Sw values, I tend to first look to see if it's realated to structure, then if BVW is remaining constant and decreasing porosity is instead driving the Sw up, and then if it could be mineralogically controlled.