To dissect this... Firstly oGCD means "off Global Cooldown", which is a skill you can use when one of your normal GCD (Global Cooldown, of course) skills are unavailable.
What can happen here is, say you have a Global Cooldown of 1.5 seconds (I picked this as a random example, let's not argue over the ACTUAL lengths of GCDs). If you have an oGCD skill that takes 1.6 seconds of animation lock, your GCD skill will have been available for 0.1 seconds before you can actually press it. This means you have have that 0.1 seconds "clipped off" from your active GCD time. Now this usually isn't a problem as, with this, it takes 15 activations for you to lose a single GCD skill.
Where the problem comes in its that is a skill takes 1.8 or 1.9 seconds to finish, you lose 0.3 to 0.4 seconds on your GCDs every time. At that rate you lose a whole GCD skill every 4 or 5 activations. If the time is even more excessive, you can lose a whole or more GCD every time you activate it. Thankfully we don't have skills that are to that extreme right now, but the activating time for Samurai is looking like it could take up at least one or more GCDs which, depending on the potency of the skills attack can be either a minor or a major DPS loss.