OBA or OBAs, stands for Optical Brightening Agents, they are chemical compounds added to the coating of some papers.They are used to enhance the appearance of colour, whitening the paper by increasing the amount of blue light reflected and, as a result, offsetting the natural yellow tones. Different papers contain different levels of OBAs, you can see if a paper contains optical brightening agents by placing a sheet under a UV light, if it lights up with blue colouring then it contains OBAs, the brighter the blue light, the more OBAs in the paper.