Got Oghma Infinium awhile back, so Septimus Signus is already dead. Cannot fix by loading a save. Can I complete the Elder Knowledge main quest without him? Or, is my main quest screwed from that point forward? I just started "A Blade In The Dark" for my main quest. Again, playing with 1.5 patch on Xbox 360, so no console or mods. Thanks for any help!

how pratell did you get the ohgma infinium without the elder scroll?

If your character already collected the Elder Scroll, then You should be fine.
If your character has not collected the Scroll yet, then get it as soon as possible, Blackreach and the Dwemer Ruins to get to Blackreach can respawn and make the scroll hard (or impossible) to get.
If you accidentally sold it to a Librarian in the COW, then You are going to have to reload an earlier Saved Game, or restart

You can get the elder scroll long before elder knowledge, and you get to keep it when you finish the oghma quest.

than no, I don't believe you will need old septimus at all.

On one of my games I had talked to him before even starting the main quest and already had the elder scroll in my inventory from the start. I had no problems, the Elder Knowledge quest was given and then automatically completed without leading the mountain

Was Septimus Signus already dead (perhaps due to Oghma quest) when you were given the Elder Knowledge quest?

So far as I know you can't inscribe the lexicon for Septimus without at least opening the crystal containing the elderscroll. So if its not in your inventory check back to where you inscribed the lexicon for Septimus.

I already have the elder scroll. My only concern is my main quest being screwed when I get to the Elder Knowledge quest because Septimus Signus is long dead. I completed the Oghma quest long ago.

The only requirement for Elder Knowledge is having the Elder Scroll.

Since Septimus must have given you the keys that you need to get into Blackreach, then you can just go back and get the scroll if you didn't pick it up first time, because you still have the keys. The whole area may have reset, but I think the room with the scroll does not. Even if it does, you don't need the cube to operate the mechanism.

Again, I already have the scroll and Septimus Signus is long dead due to Oghma quest. Am I screwed when I Reach the Elder Knowledge main quest, where you supposedly need to speak to Septimus? That is my only concern. Has anyone experienced this specific set of circumstances, and then reached the Elder Knowledge main quest? If so, were you able to complete Elder Knowledge? Anything I should know?

The only requirement for Elder Knowledge is having the Elder Scroll.

Yes, what Lord Vukodlak said. I personally went through this on my second character, who decided to do the Daedric quests first, and therefor did the quest for Septimus (resulting in his death) loooong before that part of the main quest.

What will change - when Parthuurnax tells you about using the Elder Scroll at the time rift, you simply get a dialog option along the lines of "Dude! I already got it right here, yo!" and that is that. You go straight to using the scroll. It does not break anything, it simply allows you to skip the trips to talk to this person and that person and read that book and this book and all that.

In fact, doing it in that order is now my preferred method. More efficient and I get the Oghma earlier. Win!

All that happens is Paarthurnax asks you to find the elder scroll, then you say "oh you mean this elder scroll" and the quest progresses, the septimus part if the mq quest doesn't even start.

If you did Ogham Infinatum, you will have the Elder Scroll.

So when you come to the MQ, you just say to Paathunax, I have the elder scroll, and go straight to the past.

Make sure you keep it before doing the MQ, or buy it back if it is sold, and you will be fine.

Doing the oghma Infinitum quest first is best, no running round when it comes to the MQ.

This was actually very helpful. I've encountered this problem, and now I know how to fix it. Thank you all for saving my poor Dark ELf from getting deleted.