Oatmeal - if it is indeed just rolled oats - is fine for rabbits in moderation. If they are not getting pellets, they do need a trace mineral salt block. The reddish-brown ones for general livestock are fine. They are likely getting enough fibre from the greens and bamboo but they may be a short on protein. Some alfalfa hay or alfalfa cubes might be a good idea. You will have to consider what you will be feeding them when winter comes. I would start to adjust their diet soon so that there is a seamless transition when the supply of greens begins to dwindle. My rabbits get alfalfa hay free choice, small amounts of whole grain, the mineral/salt block and as many greens as the season provides. Even in the dead of winter, I make sure they get some fresh foods.