Im assuming you mean a paper filed return....what do the Oregon instructions say?

Yes, paper filed Oregon return.

The actual Form 40 says "Important: Include a copy of your federal Form 1040, 1040-SR, 1040-X, 1040-NR, or 1040-NR-EZ. Without this information, we may adjust your return."

Form 40 Instructions Say: "14. Your federal return(s) and schedules"

And for my research, i have found a few that say just to attach page 1 and 2 of the Form 1040 and not the schedules.

CA individual return says the same thing, I always send all the pages of the federal return.    

However last time I paper filed a CA fiduciary return, the instructions specifically stated to only attach pages 1 and 2 of the federal 1041.

@BobKamman kicked me down the road once for this, but I am inclined to always send more, rather than less. When in doubt, pay the extra postage and send all pages of the federal return. Theoretically the state already has access to all of this, but ....

"obvious answer" is almost as oxymoronic as using that four letter F word with taxes. FAIR?

@George4Tacks 

Me? Kick someone down the road? The only people I kick down the road are those who can’t read instructions. Unless, of course, they are dyslexic and the revenue agency in question doesn’t furnish a comic book, and then I just shake my head and wonder why they didn't pursue a career in politics, instead.  

And I would never criticize someone for hinting that Dark Forces had spoken to them about hidden truths. (For example, “And for my research, i have found a few that say just to attach page 1 and 2 of the Form 1040 and not the schedules.” )

All I know is what Oregon tells me right there on Page 25 of its Form 40 Instructions:

“Include a copy of your federal return (front and back) with your Oregon return. Include federal Schedules 1 through 3 (if applicable), but don’t include other federal schedules unless otherwise instructed. Place it behind all Oregon forms and schedules.”

Actually, to end this discussion, I contacted the Oregon Department of Revenue directly and they want Form 1040 AND Schedules 1 through 5.  Over and Out.  Thank you. Be safe and healthy!

Call twice more, get another two answers.  People who rely on phone calls to temps during tax season always amaze me.   HUCA.  

Edited to add:

1970:  "I heard it on the phone, it must be true."

2020:  "I saw it on the Internet, it must be true."