The Lian Li PC-O5S has a the Mini-ITX form factor. Mini-ITX is even smaller than Micro-ATX, allowing PC builders the opportunity to create a low-performance, but discreet and low-power system. Using the Mini-ITX standard, the case can house motherboards and power supplies with form factors Array. The dimensions of the case itself (LxWxH) are 384mm x 465mm x 148mm.

The Lian Li PC-O5S has three expansion slots, so there is very little room for extra components, which may be a deciding factor when planning on the longevity and build type of a PC system. The case has one 5.25" drive bays, which is enough room for an optical drive but little else, and with just one 3.5" drive bay, there is basically no room at all for expansion. With two 2.5" bays, however, there is enough room for a laptop-sized HDD or solid-state drive.

Made primarily of Aluminium, the Lian Li PC-O5S is built to be strong, lightweight, and well-ventilated. This often comes at a price premium, but some see the benefits to be completely worth it. Whatever your case's material, it is important to keep a consistent airflow so that heat doesn't build up inside. It is therefore advised that you make use of the available built-in case fans and fan installation sections.