How to GREP a substring in a line which a is variable assignment?

You can use the -o ("only") flag. This command:

grep -o 'CpuIowait=[^;]*'

will print out the specific substrings that match CpuIowait=[^;]*, instead of printing out the whole lines that contain them.

If you want to use grep, try something like this:

echo "OK: CpuUser=0.11; CpuNice=0.00; CpuSystem=0.12; CpuIowait=0.02; CpuSteal=0.00;" | grep -oE "CpuIowait=[[:digit:]]*\.[[:digit:]]*"

If you're not dead set on using grep, you can pipe the input to sed:

sed -ne "s/^.*\(CpuSystem=[0-9.]*\);.*$/\1/p;"

From man sed:

-n, --quiet, --silent: suppress automatic printing of pattern space

-e script, --expression=script: add the script to the commands to be executed