From Oga’s dining link at Disney.com:
Checking In
Guests should check in no earlier than 15 minutes prior to their reservation time with the Cast Member at the end of the queue at Oga’s Cantina inside Star Wars: Galaxy’s Edge.
Each person in your party must be present upon check-in and for admission to the land and the cantina. Please be prepared to present your reservation confirmation email, photo ID and the credit card provided at the time of booking.
Guests will be admitted as soon as space that accommodates their party size becomes available. Late arrivals may not be accommodated.
Remember this ADR is limited to a 45 minute experience. I would assume the regular 15 minute grace period is in play here so if you get there by 6:15 you should be fine. Just keep in mind they really want you there by 5:45.