I have a 1 TB hard disk. But while installing KDE I aloted 50 GB for the OS partition during installation. Can I access the rest of the storage? If yes, how?

Rest of hard disk is partitioned?
if yes, it might be accessible from dolphin. Same as if it is partitioned as a Windows' partition I. e.

If no, you must create a partition and format it, using KDE Partition Manager or Gparted. If that partition will be accessible.from Wind3, format it as NTFS. If it will be accessible from Linux only, you can format it as ext4.

After it, it would be accessible using Dolphin.

otherwise, if your question is if you can add unused free space to the Linux OS (/) partition, the answer is yes, you can. Using KDE Partition Manager or Gparted too.

Keep in mind, please, that working with partitions (creating, deleting or modifying) can result in data's loss so it's recommended backuping your data.