Right ... hope this helps
Olympus, Canon, Nikon and Sony all make a decent SLR that is more than capable of taking a good picture, however there are a few things to bear in mind
Canon and Nikon are the 2 big players with the vast majority of the market, and due to this there are lots of lenses and accessories availiable both from original manafacturer and from 3rd parties like Sigma and Ta,mron for example
Sony and Olympus have a lesser share of the market, nothing wrong with them, just they dont have the high end kit in the same manor that Canon and Nikon do..
each manafacturer has their own lens mount format so a canon lens will only fit a canon etc ..... same with nikon and olympus, sony however brought the minolta lens mount so you can use sony or minolta lenses on a sony
Canon / Nikon and Sony all have the same size sensor (the bit that actualy records the image) the Olympus has a smaller one which means it sometimes creates a more noisy image but also means its longer lenses have a larger magnification than the other manafacturers
at the end of the day there is no right or wrong ... best bet is go to a shop ... hold them ... see which one feels right, then check prices of a zoom lens, somthing thats 200mm to 300mm at its long end and see if theres a massive difference in price, because if your taking pics of your dog, its not always going to be next to you, and thats where a long lens or larger zoom lens comes in
hope thats of some help ;... feel free to ask me any questions and I'll do my best to answer them
Phil