In Jain Dharma, Rishabhdev (Ṛṣabhadeva / Ādinath) is regarded as the first Tirthankar of the current time cycle, and also the first king in this era.

If by “oldest god” you mean the earliest revered spiritual figure in Jainism, then yes, Rishabhdev is considered the earliest Tirthankar.

But in Jainism, we usually do not use the word “god” in the same way as other religions. Tirthankars are enlightened beings, not creator gods. So the more correct Jain way to say it is:

• Rishabhdev is the first Tirthankar
• He is the earliest and most ancient revered teacher in the current avasarpini era

So the short answer is: Yes, Rishabhdev is considered the oldest/first Tirthankar in Jain tradition.