From your description, it doesn't seem possible. No alliance, no guarantee, no border, no great power intervention because you're not a great power.

What about your allies and Persia's allies? Are any of them great powers? That might enable intervention. Take another look at the border situation, too.

i can double check the border but the only ally that joined after Otto was Hungary who isn't one either.

Persia only borders me or my vassals and in 1 province of Mushasha which is a Mamluk vassal and former Timi vassals to the east.
Nogai aint a GP Persian ally.
My allies Hormuz and Hungary are no GPs.
Im rivalled to the Otto's if that makes a difference.

Well perhaps there's a bug. According the wiki:

The list of neighbors for any nation can be found in its diplomacy tab by filtering its relation list to neighbors only.

Does Persia show as an Ottoman neighbor when you do this?

nope will do in the future, cause i'm currently at war with no safe game prior. The warning doesn't show anymore.
This might sound weird but as Georgia is one of my vassals who does border Otto might that have a weird side effect/bug.
Both Otto and persia are kinda far appart, as in both can take a state pretty much anywhere towards eachother and they wouldn't have a border or be able to create a claim

Were the Ottomans shown in the war declaration screen? Then the text box at the top left should have mentioned why they get a call-to-arms

Would be odd for them to get a DotF cta as HK starts with the same religion as the Ottos and the Otto capital is in Europe. The only other non-obvious answer would be a co-belligerent's ally/guarantor scenario.

Maybe he enforced peace and you didn't see the popup, he should have a cb on you and persia most likely due it his missions

fyi ottomans will likely try to enforce peace on you when you attack persia if their relations are friendly enough, provided the ottomans don't have a truce with you. this bypasses the need for persia to be allied to ottomans, or guaraneed by them

Likely Enforce Peace yeah. If you miss the notification where they demand you make peace, there is no pop-up or anything telling you they've joined the war (at least not by default, there's probably a setting for it somewhere)

Kapakc님이 먼저 게시:

Maybe he enforced peace and you didn't see the popup, he should have a cb on you and persia most likely due it his missions

You know, I bet that's it, yeah. I tend to forget about Enforce Peace outside of the context of colonial nations.

I had no truces Mamluks Otto and myself rivalled each other and I owned all of QQ where Otto gets perma claims on.
the enforce peace is the only one that makes sense on speed 5 might have missed it tnx for reacting and Marquoz Paradox should pay you xD

Me? I didn't get the answer. I think Kapakc was the first to figure it out.

Because ur always on the forum trying to help people. But yeah Kapakc ty for solving this riddle.

High Society님이 먼저 게시:

Im playing as the OP mighty nation of Hisn Kayfa. How is it possible for Otto to join Persia in my war against them while neither of us are greatpowers. Otto doesn't border Persia (i'm warned), isn't allied to them and doesn't guarantee them. I can do another restart because of this (imo BS).

Probably enforce peace as others have said, I forget but if any of your vassals border them that might allow them to join the war via the warning too?