Learn Finnish: Oven leveys oli ongelma. - The width of the door was a problem.

It is built from the adjective leveä, which means 'wide'. The ending -ys (or -us) is used to turn an adjective into a noun that describes a quality or measurement, very similar to how '-th' works in English (wide into width).

Why is the word for door, ovi, written as oven here?

It is in the genitive case, which shows possession or an 'of' relationship, matching 'of the door' in the English translation. Notice that for many native Finnish words ending in -i, the final vowel changes to -e- before adding the genitive ending -n.

Why isn't ongelma in the partitive case? Don't sentences with the verb olla often use the partitive?

While adjectives and uncountable mass nouns after the verb olla (to be) often take the partitive case, ongelma (a problem) is a countable, individual noun. When stating what a specific, complete thing is, Finnish simply uses the basic nominative case.