For ongoing care, leather conditioners or a dab of sunflower/vegetable oil rubbed over the bag evenly with your palm or fingers occasionally will work to re-oil the leather and also protects the leather from cracking as well as sealing the leather for mild weather resistance. This will also help if you have got small water marks on the leather.
Scratches, scuffs, nail marks all give character to the distressed leather. Small scratches can be removed by rubbing them gently with your finger, as the natural oils on your hands will work into the leather. For larger scratches and marks apply oil over the marks with your fingers, let it soak in for a few minutes and then wipe the excess away with a clean soft cloth. This will subdue the scratch marks. Repeat the step if necessary.
Your bag will withstand an occasional rain or a drizzle on the daily commute but is not waterproofed. Avoid getting your bag wet and keep it in a cool dry place. Should it get very wet, not letting the water drops sit on the leather and drying is the key. Wipe the water away as soon as you spot it and dry it in normal room temperature. Any water marks will fade away when the bag gets dried in few days. Do not subject the bag directly onto a source of heat like a radiator or under direct sunlight to dry as that may cause the leather to crack.