Is opportunity cost is the same as marginal cost?

No, opportunity cost is not the same as marginal cost, since opportunity cost represent the expected utility loss from the highest-valued alternative given-up for an action. In this case, that not only includes marginal costs, but also fixed costs and marginal benefits foregone.

Marginal cost is the cost of producing an additional widget when you're already producing several of them. This must cover direct costs such as wages and direct overheads, but can ignore return on capital and other fixed costs.

Opportunity cost is the hypothetical loss that we would incur should we not proceed with a particular investment.

We could buy a painting in the expectation that it would rise in value. That is the value of that opportunity. If we instead to invest in widgets, the returns from them are real.

In either case, we can only buy one of the items, and the hypothetical loss from forgoing the other item is the opportunity cost of the course we chose.