Can you transfer OS from one drive to another one that's already in use without losing data?

Okay the answer to your question is yes, but you kind of have to know what you're doing.

The issue is as far as cloning programs/environments like clonezilla are concerned you do not have 200GB free on that 1TB because they operate at a disk level or partition level and/or block level. So the 1TB will have a partition allocated to effectively the whole drive. (There's no reason for it not to)

Now obviously with that in mind that means the answer is simply no. You cannot because as far as the cloning is concerned your drive does not have the required free space. HOWEVER if you know what you're doing (And this is important) you can shrink the partition on the 1TB (Though if it's a Windows based filesystem you will wanna defrag it first) as much as you can and if you get enough free space out of that (And yes you might not depending on how your files are allocated on the disk) you can then use something like Clonezilla to clone your 128GB OS drive to new partitions on that drive then expand it to fill whatever space is left.

Now the thing with that is that while you've got one drive it will be two partitions so as far as Windows is concerned it's gonna be represented as two separate drives, but your data should be in tact and you'll only have whatever free space the 128 GB partition could expand to on your OS partition. There's nothing explicitly wrong with doing this but it is kind of the cost of doing it that way. There is no practical way to reconcile the data in the 1TB and the OS drive without the use of a third drive.

All of this however is for people who know what they're doing though, if you don't you really shouldn't be messing with it because as the other person said... there is a lot of risk and you can easily lose your data if you do it incorrectly. As someone experienced in this you really never want to be doing these kinds of operations without backups or a buffer drive in case something goes wrong.