See clean-css#30 - explains what "-O" prefix is.

Author: jakubpawlowicz

README.md

@@ -109,7 +109,7 @@ clean-css-cli 4.3 introduces the following changes / features:
 -d, --debug                    Shows debug information (minification time & compression efficiency)
 -f, --format <options>         Controls output formatting, see examples below
 -o, --output [output-file]     Use [output-file] as output instead of STDOUT
--O <n> [optimizations]         Turn on level <n> optimizations; optionally accepts a list of fine-grained options, defaults to `1`, see examples below
+-O <n> [optimizations]         Turn on level <n> optimizations; optionally accepts a list of fine-grained options, defaults to `1`, IMPORTANT: the prefix is O (a capital o letter), NOT a 0 (zero, a number)
 --inline [rules]               Enables inlining for listed sources (defaults to `local`)
 --inline-timeout [seconds]     Per connection timeout when fetching remote stylesheets (defaults to 5 seconds)
 --remove-inlined-files         Remove files inlined in <source-file ...> or via `@import` statements

index.js

@@ -28,7 +28,7 @@ function cli(process, beforeMinifyCallback) {
     .option('-d, --debug', 'Shows debug information (minification time & compression efficiency)')
     .option('-f, --format <options>', 'Controls output formatting, see examples below')
     .option('-o, --output [output-file]', 'Use [output-file] as output instead of STDOUT')
-    .option('-O <n> [optimizations]', 'Turn on level <n> optimizations; optionally accepts a list of fine-grained options, defaults to `1`, see examples below', function (val) { return Math.abs(parseInt(val)); })
+    .option('-O <n> [optimizations]', 'Turn on level <n> optimizations; optionally accepts a list of fine-grained options, defaults to `1`, see examples below, IMPORTANT: the prefix is O (a capital o letter), NOT a 0 (zero, a number)', function (val) { return Math.abs(parseInt(val)); })
     .option('--inline [rules]', 'Enables inlining for listed sources (defaults to `local`)')
     .option('--inline-timeout [seconds]', 'Per connection timeout when fetching remote stylesheets (defaults to 5 seconds)', parseFloat)
     .option('--remove-inlined-files', 'Remove files inlined in <source-file ...> or via `@import` statements')