Ospreys had a fine win over Toulouse in Europe and welcome Scarlets to the Liberty Stadium with the club having ended a run of three defeats with victory over Cardiff Blues.
However, roles were reversed in September when Ospreys went to Park y Scarlets and turned around their three-game losing streak to win 23-16 over their Welsh rivals.
And with that experience in mind Tandy was desperate to avoid his team being on the wrong end of the result come Boxing Day.
"When we play at home, we expect to win," Tandy told the South Wales Evening Post. "It's going to be a huge occasion for both teams, and it's probably going to be a good environment for the Scarlets.
"We were coming in off the back of three defeats when we played at Parc y Scarlets, and derbies can change your fortunes. We'll have to be on our toes.
"The crowd's going to be fantastic so hopefully we can control our emotions.
"We know we have to be on the money against the Scarlets.
"Otherwise, we won't get anything out of the game."
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