The operating system is a vital part of your mobile device. No operating system and there's not a whole lot you can do with it. If you don't have enough storage space and you've gone through all the apps and files to see what is and isn't used or necessary, you're probably better off upgrading to a more capable phone with more storage. And for the most part, the operating system isn't an issue when it comes to your problem. Your phone's internal storage is divided into several partitions. Most are dedicated to the operating system and one big one is the user's data partition. Those system partitions have restricted access so you can't mess with them or their contents anyway. You have full control on that user data partition but not any of those system-level partitions.
Without knowing what model you have and some basic info on your storage usage (in actual numbers), there's no definitive answer to your question. Do you have a microSD card installed?