Using AO and RO values instead of AD in VCF files

Question

[Rozita]

Hello, I'm sorry if my question sounds a bit daft, but I'm confused about the difference between AD (Allele Depth) and the AO and RO values in a VCF files and was wondering if they can be used interchangeably i.e. using the AO and RO values to determine the 'counts' of the reference and alternate alleles? The VCF file that I have was generated using GATK (not by myself) and doesn't have the AD values. Thank you.