Need Some Help with Great Sword Tactics

I'm quite new to Dark Souls PvP and am having some issues combating certain weapons or play styles. I'm set on using the Obsidian Greatsword as I really enjoy the weapon and I generally enjoying STR weapons. I have been noticing that the slower swing speed of the greatsword class of weapons is leaving me more open than I thought it would. Weapons like spears, katanas and certain curved swords are proving to be a real problem for me. Any tips would be greatly appreciated; especially tips regarding spears and katanas.

First. The obsidian greatsword doesn't scale with strength, in fact it doesn't scale at all. If you're looking for a STR greatsword just about everyone would point you to the man-serpent great sword. It has great STR scaling and a great moveset.

Fighting any weapon that swings faster than yours can be problematic, as you have to be patient and pick your spots carefully Against Katanas a greatsword can either wait and attempt to trade hits whenever they attack (you should do more damage per hit and as such you'll eventually win if you keep hitting each other) Or you can try to pressure them with the threat of trading hits, running attacks 2hr1s, spaced out a little or else you'll just end up getting parried. I'm not sure if the ObsidianGS special attack has a place to be used in this particular matchup. Unless you've landed a BS/Riposte then it makes a great wake up attack.

Against spears it's pretty similar. You have to be patient, you can't just run in and swing that's a great way to get holes poked in you. Approach with your shield up, be ready to dash for them if they see the block and try to roll away after poking, you'll cover a good amount of ground. If they try to use the shield-poke while you're too close, you can simply poise tank the hit and BS them. Once you get in range to parry the fight gets easier, as they can no longer poke and roll all willy nilly. Remember this fight will typically be about patience, you might get one hit after you get in, then it's back to being patient. If they use the shield poke from medium ranges if you can instant run greatswords have a good running attack to dead angle, you'll take the poke damage but like katanas it should be less than you deal so you don't mind.

Thank you very much I'll give that a try. Should I try to play more aggressive or passive when fighting against those faster weapons? The mindset being if I play more aggressive they may make more mistakes or pick my spots as you said.

I think against katanas you can get pretty agressive, provided you have the defense/poise to trade hits favorably.

Against spears nothing but patience really works. You are being the aggressor in the exchange only it involves less swinging your weapon and more moving / forcing him to move to get into a spacing situation that doesn't favor the reach of the spear. Being about to predict rolls, and learning the instant dash attack will improve your match up against spears quite a lot

I currently have 55 poise and 320.7 physical defense on my build right now so I think I should be able to trade hits favorably especially if I can break their poise. Thank you for your advice I'll try it out when I get on XbL tomorrow night.

try to aquire 61 poise, it lets you tank 2 2h katana hits. also practice manually aiming your 2h greatsword swings.