Thank you for contacting Okta. Unfortunately we don't yet an estimated ETA /roadmap in adding Opera to the list of approved browsers, but looks like you already suggested this on the Okta Community by using the 'Suggest a feature' option at the bottom of the Okta admin console.
Features suggested in our community are reviewed and can be voted and commented on by other members of the community, therefore making it much easier for the engineering team to understand the priorities that you have for feature requests. From there, the PM team will review the top 30 most voted upon ideas each month and provide feedback/roadmap status on these via the forum.
Please let us know if there is anything else that we can help you with.
John's' recommendation to use the Chrome Extension for Opera worked for me using Opera 45.0.2552.812 on macOS Sierra 10.12.5. The Okta homepage doesn't recognize the plugin but the plugin itself works nicely.