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Originally Posted by miketoll

If you stop way down, say beyond f16 you will get diffraction effects which actually lessen the sharpness of the shot. This is true for all cameras as it is an effect of optics but my understanding is that it is more evident with digital cameras and that the smaller the sensor and the greater the pixel density the worse it is. Fortunately with a compacts tiny sensor you have great depth of field anyway so no need to stop down markedly for great DOF, indeed compacts like my old G5 will only stop down to f8 anyway. The problem for compacts is if you want shallow DOF. On my Canon 7D with my macro shots I tend to go no further than f16 although how much deterioration would actually be visible on normal viewing if I shot at f22 I do not know.
To go back to the original question: big hole = small f number eg f1.8 = shallow DOF; small hole = large f number eg f22 = greater DOF.
DOF also increases with a wide angle lens and decreases with a telephoto lens. The nearer a subject is to the camera the smaller is the DOF, the further away a subject is the greater the DOF.
The size of the sensor has an effect too, the smaller the sensor the greater the depth of field (everything else being equal) so your Olympus will be harder to achieve shallow DOF shots than a full frame camera, conversely it will be easier to obtain shots with great DOF. Hope that little lot helps!

Wow! Thank you miketoll for that little lot! I shall re-read it all and see if I can improve my pics with DOF.

Nemesis, hmmm I wonder - I haven't heard that - where did you hear it from?