It looks like the range of pricing for an O'Day 30 is in the $10,000 - 20,000 range, with the price obviously corresponding to the condition. You should make an honest assessment of your boat's condition and look at the ones for sale to to see how yours compares. I like to use fair, good, very good, excellent and bristol (or MaineSail) as my ratings for used boats. Bristol boats are extemely rare. To be in excellent condition it must not really need anything and look outstanding, also extremely rare for anything older than 20 years. My take on a price would be:
Fair - less than $10,000, maybe much less
Good - $10,000
Very Good - $15,000
Excellent - $20,000
Bristol - $20,000+