Medicare Advantage OEP vs. AEP: What's the Difference?
September 18, 2026
The Annual Enrollment Period (AEP) runs October 15 through December 7 every year and is open to anyone with Medicare. The Medicare Advantage Open Enrollment Period (MA OEP) runs January 1 through March 31 and is open only to people already enrolled in a Medicare Advantage plan. AEP lets you make broad changes, switching between Original Medicare and Medicare Advantage, changing Part D plans, or picking a new Medicare Advantage plan. MA OEP is narrower: it allows one plan change, either to a different Medicare Advantage plan or back to Original Medicare with a standalone Part D plan.
Confusing the two windows is one of the most common enrollment mistakes seniors make, and it can mean missing a chance to fix a plan that isn't working or, worse, thinking you have an option you don't. Here's how the two periods differ, who can use each one, and which one applies to your situation.
Quick Answer
AEP (October 15 to December 7) is open to all Medicare beneficiaries and allows unlimited plan changes, with the last change made taking effect January 1. MA OEP (January 1 to March 31) is open only to people currently enrolled in a Medicare Advantage plan and allows exactly one change: to a different Medicare Advantage plan or back to Original Medicare plus a Part D plan. Changes made during MA OEP take effect the first day of the month after you request the switch.
AEP vs. MA OEP at a Glance
| Feature | AEP (Oct 15 to Dec 7) | RecommendedMA OEP (Jan 1 to Mar 31) |
|---|---|---|
| Who can use it | Anyone with Medicare | Only those already in a Medicare Advantage plan |
| Switch Original Medicare to MA | Yes | No |
| Switch MA to Original Medicare | Yes | Yes |
| Switch between MA plans | Yes | Yes, one time |
| Add, drop, or switch Part D | Yes, on its own or with a plan switch | Only if you switch to Original Medicare |
| Number of changes allowed | Unlimited; last one counts | One |
| When changes take effect | January 1 | First of the month after you request it |
Why Medicare Has Two Separate Windows
Medicare created these as distinct tools for different problems. AEP exists so every enrollee, regardless of current plan, can shop the full market once a year and set up coverage for the new plan year starting January 1. MA OEP exists as a narrower safety valve specifically for people in a Medicare Advantage plan who realize, once claims start coming in during the new plan year, that the plan doesn't fit; a preferred doctor left the network, a drug isn't covered the way expected, or costs are higher than anticipated.
If you want a full picture of every Medicare enrollment window, including the ones tied to turning 65 or losing other coverage, see our guide to medicare enrollment periods explained.
What You Can Do During AEP (October 15 to December 7)
AEP is the broadest enrollment window Medicare offers each year, and it's open to everyone, whether you currently have Original Medicare, a Medicare Advantage plan, or a standalone Part D plan. During this window you can:
- Switch from Original Medicare to a Medicare Advantage plan
- Switch from a Medicare Advantage plan back to Original Medicare
- Move from one Medicare Advantage plan to another
- Join, drop, or switch a Medicare Part D prescription drug plan
You can make as many changes as you want during this seven-week window, but only the last change you submit before December 7 goes into effect. Every change made during AEP takes effect January 1 of the following year, so there's no gap in coverage between plan years. For a broader look at every window when changes are allowed, see when can i switch medicare plans.
What You Can Do During MA OEP (January 1 to March 31)
MA OEP is only available if you're enrolled in a Medicare Advantage plan as of January 1 of that year. If you have Original Medicare and a standalone Part D plan, this window doesn't apply to you at all, so you can't use it to join Medicare Advantage for the first time.
If you do qualify, MA OEP allows exactly one change:
- Switch to a different Medicare Advantage plan (with or without drug coverage)
- Drop Medicare Advantage entirely and return to Original Medicare, with the option to add a standalone Part D plan at the same time
Unlike AEP, you only get one move. Once you make a change during MA OEP, that decision is locked in for the rest of the window; you can't use it again to make a second switch. Changes take effect on the first day of the month after your plan receives your request, not on a fixed January 1 date. That means switching in January starts your new coverage February 1, while switching in March starts it April 1.
MA OEP Is Not Medigap Open Enrollment
The Medicare Advantage Open Enrollment Period is a different window from the one-time, six-month Medigap open enrollment period that starts when you first enroll in Part B at 65 or older. If you drop Medicare Advantage during MA OEP and want to add a Medicare Supplement (Medigap) policy, insurers in most states can use medical underwriting to evaluate your health history, since your Medigap guaranteed-issue window may have already closed. Review this tradeoff in our comparison of Medicare Advantage vs. Medigap for chronic conditions.
Which Window Fits Your Situation
The right window depends on what coverage you have now and what you're trying to fix. If you're comparing Original Medicare against Medicare Advantage for the first time, or you want to shop every available Part D and Medicare Advantage plan side by side, AEP is your window; it's the only one open to everyone. If you're already in a Medicare Advantage plan and something isn't working, whether it's a network change, a formulary problem, or unexpected costs, MA OEP gives you a second chance to adjust early in the plan year without waiting until the following fall.
Neither window helps if you're newly eligible for Medicare or just lost employer coverage; those situations are covered by turning 65 health coverage rules and by medicare special enrollment period qualifying events, which include job-based coverage ending. If you're navigating a transition from a workplace plan, our guide on enrolling in medicare after losing employer coverage walks through the timing.
What Neither Period Lets You Do
Neither AEP nor MA OEP erases a Part D or Part B late enrollment penalty you've already accrued, and neither one reopens a missed Initial Enrollment Period. If you delayed Part B without qualifying employer coverage, you may still owe a lifetime premium surcharge; see medicare late enrollment penalty for how that's calculated. If you missed your Initial Enrollment Period entirely and don't qualify for a Special Enrollment Period, you'd instead rely on the medicare general enrollment period deadlines and penalties, which runs January 1 through March 31 and is a separate window from MA OEP, even though the dates overlap.
Before You Make a Change
- Confirm which window applies: check whether you currently have Medicare Advantage (MA OEP) or any Medicare coverage (AEP)
- Review your current plan's Annual Notice of Change letter, mailed each September, for cost and coverage updates
- List the prescriptions and doctors you need covered before comparing new plans
- Note the effective date rules: AEP changes start January 1; MA OEP changes start the first of the following month
- Remember MA OEP allows only one switch, so compare plans carefully before submitting a change
Key Takeaway
AEP is the once-a-year window open to everyone with Medicare for unlimited changes effective January 1. MA OEP is a narrower, one-time fix available only to current Medicare Advantage members from January through March. Knowing which one you're in prevents a missed opportunity or a wasted trip to a plan you can't actually access.
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Frequently Asked Questions
Yes, but not for the same change. If you used AEP the previous fall to enroll in a Medicare Advantage plan effective January 1, you can still use MA OEP from January through March 31 of the new plan year to switch to a different Medicare Advantage plan or move back to Original Medicare, since these are separate windows tied to different rules.
No. MA OEP is only for people already enrolled in a Medicare Advantage plan. If you have Original Medicare and want to switch to Medicare Advantage, you need AEP (October 15 to December 7) or a qualifying Special Enrollment Period.
Outside a qualifying Special Enrollment Period, you'd generally need to wait until the next AEP to make changes, with coverage starting the following January 1. See medicare special enrollment period qualifying events to check whether a life event, like moving or losing other coverage, opens an earlier window for you.
Only if you use your one MA OEP switch to leave Medicare Advantage and return to Original Medicare. In that case, you can add a standalone Part D plan at the same time. You cannot add or change a Part D plan during MA OEP if you're staying in a Medicare Advantage plan, unless you're switching to a different MA plan that includes drug coverage.
No. MA OEP is about switching Medicare Advantage plans or returning to Original Medicare. The Medigap open enrollment period is a separate, one-time six-month window that starts when you're 65 or older and first enrolled in Part B, during which insurers generally can't use medical underwriting to deny a Medigap policy or charge more based on health history.