Today was my first time reviewing low-quality posts, and I accidentally pressed the "Looks OK" button a few times for posts that should clearly be deleted, as I was used to reviewing edits and "Looks OK" and "Approve Edit" are in the same place. Is there a way to reverse this decision?
No, you cannot change your decision. What you can do is:
If you meant to vote/flag (in First Post/Late Answer), just go back to the post (outside the review queue) and proceed as you wanted to initially. The end result is the same so no big deal.
If you meant to approve/reject an edit, but clicked on the wrong button, you can always go back to the post (outside the review queue) and do the edit manually (or roll it back) . Of course, this is not ideal because the acceptance/rejection has side effects (e.g., +2 rep.points in the first case, and a record of a rejected edit in the second case).
If you meant to recommend deletion, you can always go back to the post (outside the review queue) and flag it again (or vote to delete if you have that privilege). Note that three "Looks OK" votes will remove the post from the queue, so try not to miss the correct button. Conversely, if you meant to vote "Looks OK" but the post eventually got deleted, you can always flag for mod attention and explain what happened, or vote to undelete (if you have that privilege).
If you meant to vote to close/to leave open, or to reopen/leave closed, but clicked on the wrong button, you can always go back to the post (outside the review queue) and vote manually (or undo your vote). Note that three "Leave open/Leave closed" will remove the post from the queue, so try not to miss the button.
Needless to say, in most cases reviews require several votes, so one incorrect vote will not typically change the final action on the post. This is precisely what happened in your case:
where the posts were eventually deleted in spite of your "Looks OK" vote.
Note also that if you get some review wrong, you can always go to the chat and ask people to review the same item and take the right decision.