"Selling Sunset" star Jason Oppenheim is serious when he says his new girlfriend could end his confirmed bachelor status ... at least according to costar Alanna Hall .

We caught up with Alanna while she was leaving the COSF Foundation holiday event at Balmain West Hollywood Wednesday night, where she gushed about how cute it is to watch Jason be smitten with his new love -- Oppenheim realtor Jessica Vargas . Getty

ICYMI ... Jason was on Alex Hall 's "Pretty Dirty" podcast earlier this month, revealing he's planning on marrying Jess. This was huge news because he's always been adamant about how much he was against marriage.

Alanna gushed, "I've seen a change ... just his whole demeanor when he's around her and with her. I think it's really cute. I hope they get married. I th

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