For Individuals, the Form 1040 and Form 40 (Oregon) are normally due to be filed by April 15th or the next business day if April 15th is on a weekend or holiday.
For Businesses, the Form 1065 (Partnership) and the Form 1120S (S Corporation) are due by March 15th. The Form 1120 (C Corporation) is due by April 15th.
For Estates and Trusts, the Form 1041 is due by April 15th.
Exempt Organizations (Non-Profit), the Form 990 is due by May 15th.
For returns due April 15th, filing an extension allows them to file by October 15th. Keep in mind that extensions are only for filing. Tax payments due are still due by April 15th, and payments after that date make the taxes due susceptible to penalties and interest.