Hello Julien,
I haven't tried the Ouster data, but there are two options when reading messages from rosbag: you can read them in as structures or as message objects. If you use "DataFormat","struct", the read will generally be faster, but it is not currently usable with the convenience functions readXYZ and readRGB. I would suggest leaving off the "DataFormat" name-value pair to extract the messages as objects, then call the functions.
-Cam
