Your oriental planet is the last planet to rise before the sun does. That's always true no matter which house the sun is in. In your case, yes, the oriental planet is Mercury.
Since you were born right at sunrise, it's easy to see which planet is oriental: Mercury is just above the horizon, sun is coming up next, and there's nothing between Mercury and the sun. But the same rule applies to determining the oriental planet for people who don't have first house suns. If the sun is in a different part of the chart, it's not as easy to see at first glance, but you can still find the oriental planet by going around the chart in zodiacal order. The oriental planet is the last one you come to before the sun.