Learn Russian: Машина, которая возит кислород и оборудование, уже здесь. - The car that transports oxygen and equipment is already here.

возит is the multidirectional verb for transporting by vehicle. We use it here because transporting oxygen is the car's general purpose or regular job, not just a single trip happening right now in one direction. If the car were driving past you at this exact moment on a single trip, you would use везёт.

Why is the word for 'that' or 'which' которая in this sentence?

которая must match the gender and number of the noun it refers back to. Since it refers to Машина (a feminine, singular noun), it takes the feminine singular form которая instead of the masculine который.

Are the words кислород and оборудование in the nominative case?

They are actually in the accusative case because they are the direct objects of the verb возит (they are the things being transported). However, for inanimate masculine nouns like кислород and neuter nouns like оборудование, the accusative form looks exactly the same as the nominative dictionary form.

There is no verb for 'is' before уже здесь. Is it just skipped?

Yes! In Russian, the present tense of the verb 'to be' is normally skipped. You just put the subject and its location together, so Машина ... уже здесь literally translates to 'The car ... already here.'