Since Oscar is more likely to select a red piece than a yellow one, the probability of selecting a yellow piece is less than half. The best letter to represent a probability, especially one that is relatively small, is generally one of the letters used commonly for probability, such as $p$.

However, without specific values or options provided (e.g., A: 0.1, B: 0.4, etc.), a common letter used in probability questions to represent unknowns is $p$ or another variable such as $y$ for "yellow."

In summary, if you are looking for a generic letter to represent the probability of pulling a yellow piece from the bag, it would be:

$p$ or $y$, depending on the context given in the problem.

Would you like more details or have further questions?

Here are some related questions:

1. If the probability of drawing a red piece is $p_r$, how would you express the probability of drawing a yellow piece?
2. How would you calculate the probability if you knew the exact number of red and yellow pieces?
3. What does it mean when one event is more likely than another in terms of probability values?
4. If there are 3 red pieces and 1 yellow piece, what is the probability of drawing a yellow piece?
5. How would the probability change if more yellow pieces were added to the bag?

Tip: The sum of probabilities for all possible outcomes in a probability scenario always equals 1.