Assuming your USB drive is mounted under sda1:

first of all create the opt folder and mount it:

mkdir /mnt/sda1/opt

mount -o bind /mnt/sda1/opt /opt

To make the mount permanent
ARM:
wget -O - http://pkg.entware.net/binaries/armv7/installer/entware_install.sh | sh
MIPS:
wget -O - http://pkg.entware.net/binaries/mipsel/installer/installer.sh | sh