Last month, you lent a work colleague $5000 to cover some overdue bills. He agreed to pay you in 1 month with interest at 2% for the month, thus owing you $5100. Today, when the repayment is due, he asked you to extend the loan for another month and he would pay you the $5100 next month. In the meantime, you have had the offer to invest as much as you wish in an oil-well venture that is expected to pay 36% per year and a hot new IT stock that is estimated to return 48% the first year. If you let your colleague have another month, what is the opportunity cost of your decision

if you invested in the oil-well venture, you could have earned $5,100 x 36% = $1,836 in one year
if you invested in the new IT stock, you could have earned $5,100 x 48% = $2,448 in one year

You could invest in one of these options, or divide your money and invest in both options, e.g. invest $2,000 in the oil company and $3,000 in the IT company. Each different investment proportion results in a different opportunity cost.

Explanation:

Opportunity costs are the benefits lost or extra costs associated to carrying out an investment or activity instead of another alternative. Sometimes you might have several opportunity costs for one investment, e.g. invest in the IT company which is risky, invest in corporate bonds which is less risky or invest in US securities which is a safe investment.