Next week, I’m using my AA frequent flier miles to fly from the USA to St. Petersburg Russia. I’ll be flying AA’s OneWorld partner FinnAir for part of that journey. While this is one flight itinerary, I will fly AA from MIA to ORD to HEL & then Finn Air to LED on the next day (meaning I have to spend the night in Helsinki). When I redeemed my frequent flier miles for this flight, American Airlines told me the following:

1) Since my flight layover in Helsinki is over 24 hrs, I will need to pick up my bags from baggage claim at the HEL airport. I’ll re-check in my bags when I leave from HEL the next day.
2) On my flight I’m going to check-in two 50 lb. pieces of luggage. When I check-in at the MIA airport, I’ll pay American Airlines a fee for the 2nd piece of luggage. Since I’m already paying American Airlines at the airport in America, I don’t have to pay for the 2nd bag again when I re-check in my bags w/FinnAir in Helsinki.
3) Since American Airlines allows me to bring a rolled carry-on bag & backpack (or laptop bag), then FinnAir will as well.

Do you think what AA told me is true? I’m kinda afraid that when I re-check in my bags at the Helsinki airport they’re gonna charge me an arm and a leg for my bags. What are your experiences when you’ve flown 2 different airlines on OneWorld? FinnAir restricts carry-on bag weight to 17.5 lbs & AA doesn’t. Do you think FinnAir will charge me if my carry-on is more than 17.5 lbs?

By the way, I redeemed my miles for economy class flights.

When was this ticketed? Before April?

Per the regulations of the U.S. Department of Transportation, AA's checked baggage allowances would apply to your entire journey (assuming everything is booked on one PNR). I don't know about carry-ons, however. Have you tried calling FinnAir?

I am looking at doing a similiar itinerary ORD-HEL-BRU without any stopovers and all Finnair flight numbers. It seems fairly clear that AA's rules would apply on the outbound trip, but what rules apply when I check in with Finnair in BRU for the return to ORD? Like the OP, I would have 2 - 50 lb bags to check.

(I did a Star Alliance itinerary this year with similar circumstances and even though the flight numbers had the same airline but different operating carriers, when we connected to our LH flight, they made us pay an extra $60 each for the luggage before we could continue our journey. I wanted to avoid any surprises on this trip.)

Finnair rules.

FWIW, due to being an itinerary originating in the USA, the USA exeption comes into play. The baggage rule of the MSC for the Marketing carrier of the initial journey apply to the whole trip/booking.

In wich case, AY's rules are it.

Note that if ORD-HEL has been on an AA flight number, the AA's rules would have applied, even ex BRU.