There is 2 options:
1. DxO Lens Softness: by default, this setting is ideally adjust: the sharpness gain is evident without an artificial over-sharpness effect. This is one of the most impressive aspect of the software
2. Unsharp Mask: this is not useful, because this sharpness filter is too uniform, alterating the photo.

Let's detailled the advantages of those options.

The sharpness optimisation is very efficient, without over-process accentuation. In fact, the accentuation is weak at the center, where the sharpeness is generally good, but frankly stronger in the corners. Then, the sharpness is uniform and the result impressive in some photos!

On the right: RAW file (crop at 100 %) before and after the sharpness optimisation.
Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.

The unsharp mask, if used fineness can increase the sharpness feeling. But it must be used without excess. In this example, the accentuation is barely visible, exept for the leaves where it is easily visible.

On the right: RAW file (crop at 100 %) before and after unsharp mask with value Amount 1, Radius 0.5 and Threshold 4.
Canon 350D + Sigma 18-50 at 18 mm open at 2.8 on a tripod. 30 s at 800 ASA.