Tuossa is "right there". "Over there" would be tuolla.
And when the subject precedes the verb on and another word ending in either -ssa or -lla, then the article "the" is appropriate.
If the subject follows a noun ending in either -ssa or -lla, and follows the verb on, then the article "a" is appropriate. So:
Tuossa on punainen sipuli. A red onion is right there.
Punainen sipuli on tuosa. The red onion is right there.
In most contexts, 'Punainen sipuli on tuossa' translates to "The red onion is right there". And in most contexts, 'A red onion is right there/There is a red onion right there' translates to Tuossa on punainen sipuli.
Where the grammar allows, Finnish prefers to put old information before new information. Most of the time, the English article 'a' indicates new information, something being mentioned for the first time. Thus the Finnish word order location-verb-subject.
Much of the time, the English article 'the' indicates old information, something that has been mentioned in the conversation before. Thus the Finnish word order subject-verb-location.