Have same question... Did you find an answer? Thanks!
EDIT: Found the answer: https://wikis.oracle.com/display/oraclelinux/Hands-on+lab+-+Storage+Management
And you need to resize btrfs once you increase the root lv.
sudo lvextend -v -L +2G -r vg_oraclelinux6/lv_root
this will report some errors but now you do
sudo btrfs filesystem resize +2G /
Power down the VM. Use OVMM to grow the vdisk. Power up the VM again and do the usual LVM stuff to resize the partition where the VG is located on.
If this seems too weird, create another partition on the drive using the new, unused blocks and create another PV on that and add that PV to the VG. Then issue a lvresize on /dev/mapper/vg_oraclelinux6-lv_root with an resize2fs afterwards.