using 'I' in CV?

Is this okay to do? Or is it not advised?

Its not okay, using first person (“I,” “we,” “us,” “our”) is too informal.

It doesn't really matter, have always used that phrasing myself and had no issues, e.g. In this role I was asked to do this, came up against that, etc

Whats a good alternative?

I think it's fine. I have a CV that's based more on the skills I have than on just the chronological work experience, e. g. I'll have something like

June - July 2011
Receptionist at Hotel Such and Such
At this job I learned to manage time effectively as I had to attend to several tasks at the same time. I also learned how to deal with the questions and requests of clients in a friendly and efficient way.

This is just an example, but in some cases, a skill-focused CV (where you would use the word 'I') is actually better suited than a purely chronological one - for instance, if you have no work experience that's particularly relevant in itself to the job you're applying for, but you do feel you learned some things at your previous jobs that would be useful in this job.

I agree.
I have always worded things like this as well and have been fine.

Doesn't matter. Both parents are in high up director-position roles and tell me it doesn't matter; uncle recently retired early after being president of a company and told me it doesn't matter, his wife is a recruitment consultant and said it doesn't matter. I know the party-line here is that it's a big deal and you simply cannot do it, but as for me I will take the advice of people who earn over 100k a year and have been hiring people for over 25 years. If it does matter to that person hiring, you probably wouldn't want to work for them anyway - brb not hiring someone for saying "I did xyz".