Giles also showed up well in defence, sending Henry Clunies-Ross into touch with a firm tackle to end the first half.
He continued to excel in the second half, stealing the ball superbly in the tackle before running 80 metres to score.
Replacement hooker Sam Parry went over late on to grab the bonus point, but it was undoubtedly Giles' day as he was named Man of the Match.
Tandy though, was keen to downplay Giles' ever-increasing reputation and also highlighted the contribution of fellow winger Dafydd Howells.
"He's electric," Tandy said about Giles.
"Daf on the other wing has been outstanding as well. It's really pleasing for us to have young guys stepping up but it's a case of just making sure we don't get ahead of ourselves with Keelan. He's a young kid, he's professional, and I know that group in there won't leave him get too far ahead of himself, but it's about making sure he can do it consistently, week in, week out."
Ospreys sit five points clear at the Top of Pool 2 and Tandy reiterated their desire to go on and win the Challenge Cup.
"We've put ourselves in a pretty good position. We want to win as many games as we can in every tournament we are in," the former flanker said.
"Some people thought we were throwing the towel in with the squad we came out here with, but we trust our squad. It's something we are pretty proud of, the squad is in a decent place and we want to keep building momentum."
Guinness PRO12 Final 2017 Ticket Information: Fans can save up to 20% on selected tickets, and prices start at just €30 adults and €5 child, and be booked via www.ticketmaster.ie. For further information visit http://www.pro12rugby.com/final
Guinness PRO12 Ospreys News
63x46.png){kind=logo}