Hi JssyWang,
welcome to the Apple (user-to-user) Community!
Re: "Is Apple watch Ultra 2 compatible with my oticon hearing aids?"
Wondering if your Oticon Real hearing aids are made specifically to work with Apple products
Eg:
Hearing Aids listed as "Made for iPhone" (MFi) hearing aids are compatible with iPhone and iPad.
While those marked with an asterisk (*) are also compatible with Mac
The List of Made for iPhone hearing devices - Apple Support
lists only one Oticon Real model...
- Oticon Real X* as being compatible with iPhone, and iPad (and Mac)
seems they use *bidirectional streaming, which enables you to talk hands-free.
Of course, there are hearing aids that are not "MFi" that can still work with Apple Watch
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Re: ... "whether it's possible to stream a phone call answered on my watch directly to my hearing aids."
As far as I know the iPhone (and iPad ?) are capable of switching the audio to hearing aids during a call.
Apple's Watch does not seem to have that capability.
Am wondering if you use an Oticon app on your iPhone, and whether or not the Oticon app could be helpful.
Oticon's website says there is a new app ...
https://www.oticon.com/support/wireless-listening/oticon-companion-app
I do not know what features the new app may have.
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For more support, if needed:
Apple's Accessibility Support webpage (info / support / contact options, etc.)
Apple Watch Support webpage: (info / support / contact options, etc.)
Apple Watch - Official Apple Support
or
Apple phone numbers around the world:
Contact Apple for support and service - Apple Support
All the best :-)