The Andis is much lighter weight of the two at approx 1/2 lb. when weighed while supporting the cord off the scale. The Oster weighed approx 3/4 lb. The Andis grip at the widest point is 5 1/2 inches around and the Oster is 6 1/8 inches. This alone makes the Andis easier to manuever in my somewhat smaller female hands. The Andis is much, much quieter and would be better for shy dogs or used for first time on a dog that had never been groomed before. Also, I think I will prefer it for around the face and trickier tight areas like the arm pits. Neither seemed to heat up quicker than the other.
The Oster does seem sturdier and more workhorse like of the two and it's motor noise, while louder, did not sound loose or less well made. To the contrary, it has a solid sound and feel. Its on/off and speed switch is at the base near the cord and cannot be accessed as easily while clipping. It should be noted that I discovered it does not universally accept the blades I have that are Andis brand. The Oster works with most of the Andis blades, but the steel ultraedge 30's did not fully load on the tongue of the Oster as their slot was a teensie bit too tight. The Andis accepts Oster blades fine. In fact, to keep blades from getting loose on one or the other I will elect to keep certain blades for each clipper I think.
Neither manual is exceptionally good. I changed the drive assembly on the older Andis clipper I had to try to "fix" whatever it's problem was that was causing blades to seize up (prior to tossing it) and had to use a magnifying glass to look at the instructions and my eyes are not in bad shape. It did not fix the problem and hence I bought the new clippers. The newer Andis has the same instructions. The Oster came with 2 books and one for maintenance appears to have been written in the 1940's for a much older model. There was both oil and grease. I carefully read the whole wad of instructions and after several times through deduced my model had NO grease holes where indicated. I really haven't determined where I should put that grease yet. Hopefully my friend, a professional groomer, may be able to enlighten me.
I clipped Ollie and he shied away from the louder clippers at first but quickly relaxed. The Osters are MUCH better at cutting his thick curly side and back hair and power through there quicker and easier with fewer swipes to do a clean job. I am glad I have the Andis for ears and face and those tight areas though. Also, the Oster's being heavier would tire me out more if I were grooming several dogs. In summation, if you have big hands and a VERY heavy coated dog or maybe are grooming matted rescues, an Oster might be a better choice. If you are a smaller handed person with lighter coated dogs or groom frequently so there is less heavy duty clipping and if you have shy or frightened dogs, maybe the Andis would suite you better. Frankly I am glad to now have 2 clippers so one can cool while I use the other. I can power through the heavir coat areas and grab the Andis for the face etc. Mind you my opinion is just that and you may feel differently. I am not financially associated with either brand.