Unfortunately, changing the default "AND" to "OR" relation in filter is not possible. This is a problem when the number of filter conditions is big and makes a filter creation very unconvenient. I have succesfully tried a method to change "AND" into "OR" in filter that is fully created. You can copy the filter using Ctrl+C method in filter tree and then paste it in ordinary text editor (e.g. Notepad). As you can see, the filter code is pure text, where you can change some elements.

Among others, you have there following sequence:

The magic digits in "Filter" block decides about filter behaviour, so in the example above the condition is read as "Sender is...". When more conditions are added then they are separated by appropriate three digits with apostrophes sequences. The following code means two conditions with "AND" operator:

Filter: {\0D\0A\20`0`4`<this is a condition #1>\0D\0A0`0`4`<this is a condition #2>\0D\0A}

If we want to use "OR" it is sufficient to change "0" to "1" as in example below:

Filter: {\0D\0A\20`0`4`<this is a condition #1>\0D\0A1`0`4`<this is a condition #2>\0D\0A}

When the code of filter with such changes will be pasted into filter tree, you will have "OR" instead of "AND" in conditions.