Do I have to buy a plan by the end of open enrollment (i.e., for 2018 coverage, by December 15, 2017) to avoid a tax penalty?

Individuals with no coverage | Individual Responsibility Requirement (Individual Mandate)

Yes, unless you can get health insurance elsewhere, for example, through your employer or you are on Medicare or Medicaid. Outside of open enrollment, you will not have another chance to enroll into marketplace coverage unless you experience a life event qualifying you for a special enrollment. See Open and Special Enrollment Periods. (26 U.S.C. § 5000A (a)).

Individuals with no coverage
Individuals with coverage
Coverage for small employers
Post enrollment issues