Learn Finnish: Festivaali oli mahtava. - The festival was awesome.

oli is the past tense form of olla (to be). The verb olla is a Type 3 verb, not a Type 4 verb, and its past tense is irregular. Instead of taking the -si- marker, its root simply changes (on becomes oli, olen becomes olin). You will only use the -si- marker for Type 4 verbs like haluta (to want) which becomes halusi (wanted).

Why is the adjective mahtava in the basic nominative form instead of the partitive case (mahtavaa)?

Because festivaali is a specific, countable noun. In Finnish, if you are describing a whole, singular, countable thing as a subject, the descriptive adjective stays in the nominative case. You would use the partitive case (mahtavaa) if you were describing an uncountable mass noun, like water or weather, or an abstract concept.

Is festivaali a normal Finnish word, since it sounds exactly like the English word festival?

It is a recognized loanword. Finnish adopts many international words by simply adding an -i to the end and adjusting the spelling to fit Finnish rules. The letter f is not native to traditional Finnish, so you will mostly see it in relatively modern loanwords like this one.