Main function: calendar for reserving, list/map view of available restaurants nearby, search by filters, restaurant profile. App would exclude the "points" / loyalty program features.
I own a mobile app develop agency for the last 15 years. Few other requested apps like opentable before.
To create the main functionality of opentable - web app, mobile app, main management panel, restaurant app) from scratch it would take around 2500-3000 ours of coding. this is based on estimate one year back and assuming you are starting from scratch.
If you were to customize existing platforms like iScripts UberforX it would be closer to 500-750 hours.