Can I use the Medicare Advantage OEP if I have Original Medicare?

No. The Medicare Advantage OEP is exclusively for people already enrolled in a Medicare Advantage plan. If you have Original Medicare, this period does not apply to you; your main opportunity to enroll in Medicare Advantage is AEP.

AEP Preparation

AEP vs. Medicare Advantage Open Enrollment Period: What's the Difference

Q: What is AEP?

The Annual Enrollment Period (AEP) runs October 15 through December 7 each year. During this time, anyone with Medicare can switch Medicare Advantage plans, return to Original Medicare, or enroll in, switch, or drop a Part D drug plan. Changes take effect January 1.

Q: What is the Medicare Advantage Open Enrollment Period (OEP)?

The Medicare Advantage OEP runs January 1 through March 31 each year. It is only for people already enrolled in a Medicare Advantage plan, and it lets them switch to a different Medicare Advantage plan or return to Original Medicare once during that window.

Q: How many times can I switch plans during the OEP?

Only once. Unlike AEP, where you can change your mind multiple times before December 7 and only the last choice counts, the Medicare Advantage OEP allows a single change between January 1 and March 31.

Q: What if I pick the wrong plan during AEP?

If your new Medicare Advantage plan takes effect January 1 and isn't working out, the Medicare Advantage OEP gives you a second chance between January and March to switch to a different Medicare Advantage plan or return to Original Medicare.

By Susana Marcos, Licensed Medicare Agent · Published August 4, 2026 · 6 min read

ES This article is also available in Spanish. Read it in Spanish →

If you've ever heard someone mention "the Medicare enrollment period" and wondered exactly which one they meant, you're not alone. Medicare has several enrollment periods with similar-sounding names, and two of the most confused are the Annual Enrollment Period (AEP) and the Medicare Advantage Open Enrollment Period. They sound almost identical, but they follow different rules, apply to different groups of people, and serve different purposes.

With AEP opening October 15, this is a good time to understand the difference before the busiest season of the year arrives.

Two Periods, Similar Names, Different Rules

The confusion makes sense. Both periods allow Medicare Advantage related changes, and both have fixed dates every year. But one is open to nearly all Medicare beneficiaries, while the other is exclusively for people who already have a Medicare Advantage plan. Understanding which one applies to you avoids rushed decisions and helps you know when you actually have the option to switch.

Annual Enrollment Period (AEP)

October 15 through December 7. Open to anyone with Medicare. You can switch Medicare Advantage plans, return to Original Medicare, or enroll in, switch, or drop a Part D drug plan. You can change your mind multiple times; only your last choice counts.

Medicare Advantage Open Enrollment Period (OEP)

January 1 through March 31. Only for people already enrolled in a Medicare Advantage plan. Allows a single change to a different Medicare Advantage plan or back to Original Medicare. Does not allow enrolling in Medicare Advantage for the first time.

What Is the Annual Enrollment Period (AEP)

AEP is the period most people know about. It runs October 15 through December 7 every year, and any changes you make take effect the following January 1. During AEP, you can move from Original Medicare to a Medicare Advantage plan, from one Medicare Advantage plan to another, back to Original Medicare, or change your Part D drug plan. It's the most flexible period of the year because you can change your mind as many times as you want before the deadline; Medicare only applies your most recent decision.

To see what options you have available during AEP, check out our Medicare Advantage in California guide, or see our Annual Enrollment guide for the full calendar.

What Is the Medicare Advantage Open Enrollment Period (OEP)

The Medicare Advantage OEP is narrower and less well known. It runs January 1 through March 31, right after the changes from AEP take effect. Only people already enrolled in a Medicare Advantage plan can use it, whether they've had the same plan for years or just switched during the previous AEP. During the OEP, you can make one change: move to a different Medicare Advantage plan, or return to Original Medicare (and add a Part D plan if needed).

This period exists mainly as a safety net. If you picked a new plan during AEP and by January realize a doctor isn't in the network, or a medication isn't well covered, the OEP gives you a chance to correct that decision without waiting until the next AEP in October.

Key differences between AEP and OEP:

AEP: October 15 through December 7. OEP: January 1 through March 31
AEP: open to all Medicare beneficiaries. OEP: only for those already on Medicare Advantage
AEP: you can change your mind multiple times. OEP: only one change allowed
AEP: includes Part D plan changes. OEP: does not allow standalone Part D changes

Who Can Use Each Period

If you have Original Medicare and want to explore Medicare Advantage for the first time, AEP is your main window. If you already have a Medicare Advantage plan and just want to check whether it's still your best option, you can use AEP to compare all your options calmly, or wait and use the OEP as a more limited second chance if something isn't working out after January.

A special case: if you qualify for both Medicare and Medi-Cal, you may have access to a Special Enrollment Period that lets you switch plans monthly, outside of these two windows. See our dual eligibility guide to see if this applies to you.

Why This Difference Matters for You

Understanding which period you have access to prevents two common mistakes: assuming you can switch plans anytime during the year when you actually can't, or letting AEP pass by assuming the OEP will give you the same options later. The OEP is a limited safety net, not a second version of AEP. Planning your decision during October and November, with time to compare calmly, remains the best strategy.

Reviewing this in Spanish, or helping a family member who prefers it? Our sister site Medicare California covers the same enrollment periods in Spanish.

Want help figuring out which period applies to you?

A licensed agent can confirm which period you're in, compare your options, and walk you through it all at no cost, no pressure.

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Frequently Asked Questions

What is AEP?

The Annual Enrollment Period runs October 15 through December 7. Anyone with Medicare can switch Medicare Advantage plans, return to Original Medicare, or change their Part D plan, with changes effective January 1.

What is the Medicare Advantage Open Enrollment Period (OEP)?

It runs January 1 through March 31, and is only for people already enrolled in a Medicare Advantage plan. It allows a single change to a different Medicare Advantage plan or back to Original Medicare.

Can I use the OEP if I have Original Medicare?

No. The OEP is exclusively for those already on Medicare Advantage. If you have Original Medicare, your main opportunity is AEP.

How many times can I switch plans during the OEP?

Only once, unlike AEP, where you can change your mind multiple times before December 7.

What if I pick the wrong plan during AEP?

The Medicare Advantage OEP, from January through March, gives you a second chance to switch to a different plan or return to Original Medicare.

One More Thought

Knowing which period applies to you, and when, is the difference between feeling in control of your coverage and feeling stuck with a plan that no longer works. AEP gives you the widest window to compare calmly; the OEP exists as a backup, not a main plan. Starting to think about this now, in August, leaves you plenty of time before October arrives.

Share this with someone you love. If a parent, spouse, or friend doesn't know the difference between these two periods, this guide could save them a costly mix-up this fall.