If you do not set any custom ranking criteria then they will be ordered by objectID in DESCENDING order by default, and it is not possible to change this behaviour. However, we recommend adding a custom ranking attribute whenever possible.
By default, upon an empty query, the search results will be based on the ranking criteria: https://www.algolia.com/doc/guides/managing-results/relevance-overview/in-depth/ranking-criteria/ (assuming you have not made any changes to your ranking criteria).
As all results will match equally for relevancy they will be ordered by any of the custom ranking criteria you have set on the index: https://www.algolia.com/doc/guides/managing-results/must-do/custom-ranking/#custom-ranking
Here are 2 ways in which you can use a workaround to achieve the functionality of ordering by ObjectID:
1 - You could add a new attribute to your records, and pass the same value of your objectID to it. Then, you could use it for your ranking strategy, and define it as your last custom-ranking criteria. We recommend using boolean or numerical values for the custom ranking. However, if you use string values, please remember that they are sorted by lexicographical order. More information about that: https://www.algolia.com/doc/guides/managing-results/must-do/custom-ranking/how-to/configure-custom-ranking/#string-and-null-or-absent-values
2 - To display records in alphabetical order with empty search, you can use a replica - with the same data in your replica index, you can use different sorting strategies using the sortBy attribute. You may also want to look into Sort an Index Alphabetically.