"I often swim alone."

Translation: Gyakran úszok egyedül.

[Bukino]

why is there - om in - ok's stead?

[MrtonPolgr]

exactly because of what the other comment has mentioned. -ik words tend to accept definite-looking ending for singular first person, it can even sound more common.

[DonFry]

I thought uszni was an -ik verb, so it always conjugates to uszom, not uszok despite the lack of a definite article. Yet, the answer here says both are correct.

[vvsey]

This rule is not exactly set in stone. There are many exceptions and regional variations. Some verbs are acceptable in both versions, others sound really weird using one or the other. There are also "fake" ones that the rule does not apply to at all. And there are ones where the two versions may be used in different contexts.
So, they may have thought it good to accept both versions here.