We have a olympus slr

You will need to switch the camera to manual exposure mode.  You will need to try some different exposure, but with the moon half full, you should probably get good results with somewhere between 1/60 and 1/500 sec at ISO 200  It will depend on the f/x.x number of your lens as well.  If the exposures are washed out you need to increase the shutter speed (higher number on the bottom of the ratio i.e. 1/1000).  If the exposure is too dark you will need to decrease the shutter speed (i.e 1/30).  You also need to make sure that you don't shake the camera when you press the button.  It is best to mount the camera on a tripod, if you don't have one try bracing it against a large solid object.
Note that in automatic mode the camera will over-expose the moon with a longer exposure.  The longer the exposure the more likely you are going to blur the image.  Also note that the more you zoom in, the more blurring you are likely to have.
I hope that helps.