Below is a step by step of how I was able to resize an existing pun and ASM disk to a new size.
Oracle recommends for ONLINE disk group space increase to actually create a NEW LUN, and add it to the existing disk group. There have been a few bugs w/ ASM recognizing. If you do wan’t to attempt the resize method, you can attempt the instructions below on a test system. Things like this are why I typically recommend NFS. No partitions, no filesystems, none of the issues.